Elegantly simple collaboration tool of 37signals.

Before complaining it's lack of functionality, think twice it is really necessary. Don't hurt your project with complex project management tool.

If you are an heavy bookmarker, Diigo is for you.

Diigo support bookmarking, annotation( and sharing), public/private mode, list(something like category), tagging, and groups.

If you have not used it, I recommend you to use it.

Springloops gives you subversion repository, and uses Basecamp as issue tracking system.

If you are using basecamp, springloops is good for your source code management and issue tracking.

It also support easy-to-use deployment tool, but I'm using Capistrano for that purpose. But if you are programming in PHP etc, deployment functions will be helpful.

I think twiddla is a very promising service. I support online annotation, sketching, and even real time audio chats. And All these things work in the web browser without plugin. This is somewhat similar with the conceptshare and sharedcopy.