Google Docs – good for online document storage, collaborative writing, off-site backup
Evernote - note-taking facility available via mobile, online and offline with synchronization available between any installation / version
Scribd - share documents which are automatically converted so they can be downloaded as Word, Adobe pdf or .txt files / can be embedded in HTML pages
Wordle – useful for analysis of a topic or issue by simply uploading the text you want to look at
Zoho - full suite of tools from Office-type word processors to note-taking and project management tools. Free option available on most tools.
Blogging / RSS / Collaboration
Blogger - good, free blogging service from Google
Delicious - one of the original social bookmarking sites. Straightforward to use!
Diigo - great social bookmarking tool with added annotation, communities and discussion. Can also automatically post your links to Delicious if you've already got a Delicious account you don't want to lose.
DimDim - free web conferencing / web-hosted online, open-source meeting tool
Drop.io - instantly create collaborative working spaces, private, no account necessary, shareable outputs, easy to use.
Google Reader - full-featured RSS aggregator with good integration into other Google services
WordPress - another blog service, but is slightly more 'professional' looking and the ability to view stats on blog postings is also useful
WetPaint - free wiki with plenty of templates and features
Video / audio
Jing - web-based screencasting tool, create screen captures, narrated video etc.
quietube - view or share YouTube videos without distracting comments and other YouTube branding
ScreenToaster – create and record video to create screencasts or tutorials
SlideShare - useful for hosting PowerPoint slides to share / embed, also good for linking in MP3 files to easily create a slidecast
Xtranormal - create 3D videos by selecting characters and inputting the script. Sharable video output.
YouTube - video sharing site with lots of creative commons licenced resources / facility for comment / closed captioning
Facebook - major social networking site, useful for both personal and business networks / use of groups for students etc
LinkedIn - similar to Facebook, but with a more professional feel
Twitter – microblogging tool, useful for concise communication, resource-sharing, networking, discussion, current affairs / breaking news
Exploratree - from Futurelab this is an online ideas tool, a little like mindmapping but with other types of 'thinking' diagram templates available
Gliffy - creates flow charts, organisational charts, plans etc in the style of Microsoft Visio and can support collaborative working
MindMeister - mindmapping tool, allows users to create up to 6 mindmaps for free and export in various formats including jpg, rtf etc
ProjectDraw - CAD-like online drawing tool
Skrbl - free, shareable online collaborative whiteboard.
Flickr - online photo sharing (plus tagging, commenting and annotating) and photography-related discussion forums / community
Picnik - online image editing with links to popular image-hosting sites such as Flickr, Picasa Web Albums, Facebook etc
Miscellaneous / Utilities
Box.net - online file storage service
Doodle - meeting scheduling toolDownForEveryoneOrJustMe - does what it says! Put in a URL to check whether or not a site is down or it's just you having problems.
How Do I? - search Google for tutorials on any subject
Media Convert - convert files from one format to another using this free tool
Read The Words - create MP3 audio files from text input with a variety of voices and speeds to select
This list is a work in progress, so watch this space (i.e. I'm sure I've forgotten some of the ones I really like and I may sneak back to edit them later!) :o)