Google Docs & Spreadsheets - One of the more developed online office tools, complete with interfaces that match Word, Powerpoint, and Excel.
Zoho Office Suite - Zoho is one of the most complete online office suites, offering more tools than you'll even find in Microsoft Office's student and teacher version.
Etherpad - Online word processor that lets people collaborate and work in real time. (Really simple interface--one of my favorite tools!)
EditGrid - EditGrid only does spreadsheets, but does them very well.
Open Office - Open Source Productivity Suite that works virtually like Office.

Notetaking:

NoteMesh
ShortText
Yahoo! Notepad
YourDraft
Stickies
NoteCentric
Evernote - Use Evernote to save your ideas, things you see, and things you like. Then find them all on any computer or device you use. For free.
TrackClass - take notes, track classes and assignments (and set text message reminders). (free)

Flashcards:

(From Technology for Teachers )
Flashcards are one of the simplest and most common ways to study for a test. Free Technology for Teachers has featured numerous flashcard resources in the past. Here are ten good places to create and find flashcards online.

Cramberry allows users to share flashcards with other users. After creating your flashcards you can contribute to a public gallery of flashcards. If you don't have time to make flashcards of your own, you can search for and study flashcards in the public gallery. To use the flashcards you will need to create a Cramberry account. For your students that have iPhones (none of my students have one) Cramberry now has an iPhone app called Flash-Me.
Flashcards created using Funnel Brain can include a "third side" to a flashcard. A side that contains the explanation for the answer to a question. In addition to text Funnel Brain flashcards can include videos, audio, and pictures. Under your videos and images you can type your question, answer, or explanation. Just as with many other flashcard services Funnel Brain flashcards can be shared with other users.
Ediscio is a little different from a lot of flashcard creation tools because Ediscio lets you build sets of flashcards individually or with the help of other Ediscio users. The other distinguishing feature of Ediscio is that flashcards can include images. (Ediscio is also featured in Twelve Essentials for Technology Integration).
Cobocards, like other flashcard services, allows you to create customized sets of flashcards. One of the key differences between Cobocards and other flashcard services is that Cobocards provides you with pdf copies of your flashcards that you can print to study offline. Of course, you can study also study your flashcards online.
Flashcard DB is one of the most well-designed free flashcard programs that I've seen on the web. Flashcard DB, like most flashcard programs, allows you to browse existing sets of flashcards or create your own custom set of flashcards. What makes Flashcard DB different from its competitors is the study methods that can be used with the flashcards. Flashcard DB allows students to study using the Leitner System or the Graded Space Repetition Method.
On Quizlet you can make your own flashcards or study from publicly shared sets of flashcards. Quizlet offers you the ability to study your cards in five different ways including playing a couple of games with your cards. When creating your flashcards you can enter information for each card individually or import information from a text document to be used in your card set. If you don't have the time to create your set of flashcards, browse the publicly shared flashcards.
Study Stack is one of the better review game creation tools that I've tried. Study Stack allows teachers and students to create flashcards, crossword puzzles, matching games, word searches, and other classic study games for any subject area. You can create a game using any type of numerical or text data. Once you're data is in your account, you can use that data to create multiple games.
On Muchobeets students can create their own customized "stack" of flashcards. For students preparing for a general exam like the SAT,Muchobeets has a good collection of publicly shared vocabulary flashcards. The Muchobeets service is very easy to use. If you want to make your own flashcards simply register with your email address and get started using the very intuitive flashcard creation template. No registration is required if you want to use the publicly shared stacks of flashcards.
Knowtes is a great resource for finding or making all kinds of flashcards. The Knowtes library is full of ready to use decks of flashcards covering subjects appropriate from Kindergarten through college. The option to download a library of flashcards is the best feature of the Knowtes program. Students who have laptops can download flashcards and use them to study without having to find an Internet connection. For example, in the district where I teach every middle school student is given a laptop, but many students do not have Internet access at home. Being able to download study tools is very useful for those students.
Flashcard Flash is a handy little search engine designed for one purpose, helping you find sets of flashcards. Flashcard Flash was built using Google Custom Search. Flashcard Flash searches six different flashcard services including Flashcard DB, Quizlet, and Study Stack all of which I've previously reviewed and found to be excellent services.

Mindmapping:

Mind maps are a graphical method of taking notes. The visual basis of them helps one to distinguish words or ideas, often with colors and symbols. They generally take a hierarchical or tree branch format, with ideas branching into their subsections. Mind maps allow for greater creativity when recording ideas and information, as well as allowing the note-taker to associate words with visual representations. Mind maps and concept maps are different in that mind maps focus on only one word or idea, whereas concept maps connect multiple words or ideas.

Online Study Help:

ChaCha - Who? What? When? Where? Why? How? - Can be used on your mobile phone too!
Shmoop - Loads of resources around ELA, Social Studies, and Music.
Wikipedia - Online encyclopedia with tons of resources. It is as accurate as print encyclopedias (still with a margin of error, though) but is a good starting point for research. Good opportunity to practice critical thinking and determining importance skills.
Yahoo! Answers - When searching the web fails, someone on Yahoo! Answers may be able to show you where to find the information you're after.
SparkNotes - SparkNotes are (mostly) free, online CliffsNotes for a large number of books. They also do test prep, mathematics, science and a number of other subjects. Of course they can't really substitute for actually reading a book, but they can help you if you're having trouble figuring out Emily Bronte. (And it turns out that many CliffsNotes are now online for free as well!)
Google News - Google News, especially with their new archive search, can be an invaluable research tool if you're researching a recent historical or current event.
CNN.com - News source - breaking news, world news, etc.
Study Skills Online - Learn how to become a better learner and how to manage your own learning.
Study Guides and Strategies to help with studying efforts!
Web Apps for Students from Mashable.com
100 Ways to use your iPod to help you study!
MORE STUDY APPLICATIONS

Bookmarking:

Delicious - Social Bookmarking tool that lets you save your bookmarks to access on any computer and share easily with friends.
Diigo - Another Social Bookmarking tool that does what Delicious does, PLUS allows sticky notes on web pages, website annotation, group lists, and easy output to blogs, Twitter, etc.
Social Bookmarking Tutorial Wiki

Collaboration:

Stikipad - A collaborative wiki service that you can use to keep track of group notes on a project.
Scribblar - online whiteboard to collaborate with others. (free)
Dweeber - study together with collaborative workspaces, chat, and more. (free)
Students can also use collaborative technologies like Google Docs and Etherpad (listed above).
MORE COLLABORATION APPLICATIONS

Calendars

Juggling your class schedule, extra curricular activities, study time, and social life can be a challenge. The calendar apps below might help.

Calculations

Web applications to help you do calculations.

Citation Makers:

EasyBib - A tool to take the pain out creating a bibliography.
OttoBib - Enter the ISBN of a book, and automatically have your bibliography entry created in MLA, APA, Chicago, BibTeX, or Wikipedia style.
Zotero - A Firefox extension that lets you "collect, manage, and cite your research sources" from within your web browser.
Citation Machine - Respect the time and creative effort of those whose information you use.

Helpful Websites

Applications/Tools/Resources to help students with Studying Efforts!

Word Processing: