37Signals.com - Maker of collaboration tools including Basecamp (others listed below).
8apps.com - A mixture of social network and productivity applications.
BlueTie.com - Online collaboration directed towards small and medium sized businesses.
Businessitonline.com - Centralized cash flow, documents, calendars and more for a team or small business.
CentralDesktop.com - A full work suite for project teams including spreadsheets, file sharing, calendar and more.
Colligo.com - Allows you to work on projects off-line and then sync them when you can login.
ConceptShare.com - Share concept designs and allow invited workers to mark-up, comment, and give feedback.
Confluence - An enterprise-class wiki with features such as PDF exporting.
ContactOffice.com - Allows groups to share documents, calandars, contacts, and files. They can then access them from a number of different mobile devices.
Coventi.com - Upload your documents, share, and allow others to highlight portions or edit.
Copperproject.com - Share projects, tasks, documents and timelines with your entire team.
CrossLoop.com - Allows two computers to connect securely to share their desktops for collaborative work and transfer of files.
Eloops.com - Share files, calenders, projects, and then create RSS feeds to share your public ones.
Foldera.com - Allows for communication over projects, and centralizes work to make sure version is always current.
Glide - Facilitates group meetings to review projects together.
Google Docs & Spreadsheets - From the behemoth known as Google, collaborative documents and spreadsheets.
GroupSharp.com - Allows you to share lists, databases, file sharing, and wikis for your team.
Huddle.net - Designed for collaborative work in the fields of PR, advertising, marketing, design and more.
Instacoll.com - Allows real-time sharing of documents to work on them with colleagues.
Joyent.com - Get collaborative software and backup services in one package.
Longjump.com - Offers standard collaboration, but also the ability to build new applications for your team to use and even market.
Mindquarry.com - Work on projects online or off-line, then synchronize as soon as you are back.
Near-Time.net - Build wikis, blogs,create documents for your team to work on, and have the ability to sell your content and expertise.
Nexo.com - Share files and notes amongst your company, or even with other companies you invite.
Octopz.com - Allows your team to work a wide range of documents including video, audio, and flash animations.
OpenTeams.com - Allows your team to create cPages of your project.
QuickBase.com - A collaborative tool from Intuit, the makers of Quicken.
PlanHQ.com - Specifically for groups working on business plans.
ProjectSpaces.com - Share and work on documents, notify team members via RSS and email.
Solodox.com - Create, access, edit, share, and collaborate on documents & projects.
TeamWorkLive.com - Track tasks, centralize communication, share documents and files, then collaborate with clients and remote teams.
TheOpenDoc.com - Create multiple workspaces with multiple documents, then share them for group editing.
Thinkature.com - Collaborative workspace with voice chat and numerous drawing tools.
Tracbac.com - Workspace for your group that features versioning, IM, VoIP ad more.
Uhroo.com - Focuses on working on goals, estimates, client feedback and more.
Vyew.com - Your team can work on a project and then publish it directly to where you choose on the web.
WebOffice.com - Work on documents and more with live web meetings.
Wrike.com - Project management that allows you to assign tasks and then let people work in teams on them.
Writewith.com - Create documents and invite others to edit and chat while you do so.
Yugma.com - A catch-all collaboration site for files, tech support, presentations and more.
Zimbra.com - Email service with collaboration tools.
Zoho.com - Create documents and grant permissions for others to work on an entire page or just one portion or object.
Kalabo.net - Site for musicians to collaborate on compositions together.
Freepository.com - Specifically for computer program development and allows storage of code to be worked on from anywhere.
Glypho.com - Site for group writing of fictional stories.
Novlet.com - Collaborative writing of non-linear stories in any language.
WebBrush - Focuses more on graphical sharing and includes drawing tools.
Writeboard.com - From 37signals, perfect for collaborative writing, journalists, bloggers and more.
WriteMaps.com - Designed specifically for creating and sharing sitemaps amongst web teams.
Family and Social Collaboration
Cozi.com - Helps families organize and communicate in a web based method.
Famundo.com - Built for families or small groups to organize and share contacts, calendars, and projects.
Grouptivity.com - Uses email to start a group discussion on any subject of your choice.
LooseStitch.com - Create to do lists and share them with whomever you choose.
Mecanbe.com - Post your goals in life, get suggestions on how to achieve them.
Stixy.com - Share documents, photos, to do lists and more with family and friends.
Wamily - For teams, families, clubs and everything in-between. Share photos, run a wiki, have conversations.
Bubbl.us -Create and share your mind map with your team, also embed in to your website or blog.
Comapping.com - Mind mapping for everyone from students working together to project management.
Gliffy.com - Draw & share mind mapping diagrams.
Kayuda.com - Mind mapping for individuals or organizations.
Mind42.com - Mind mapping for one, or multiple users.
Mindomo.com - Has both free and premium accounts for mind mapping.
Mindmeister.com - A mind mapping tool you can share with an unlimited number of simultaneous users. (via)