You might wanna try redmine (as an alternative to trac). It has a wiki, news updates, file uploads, with plugins you can do twitter like status updates, besides all the project management things. Look at the plugins list on their site. You can create unlimited projects.
Although it might not be a true collaborative tool in the sense you mean, but it helps you to do all the other stuff that an early stage startup needs. I've tried more than a dozen hosted solutions and finally gave up and installed redmine for all project related needs.
ohh and Dropbox allows you to undo file changes upto 30 days in the past. I think they do have something called pack-rat, but i don't know it's pricing.
Surprisingly, the microsoft small business options were interesting. With a plugin you can use your desktop office to update online docs. Which is a killer feature in my opinion. But i never got around to testing it, since we'd started on google apps. Their live mesh is similar to dropbox, with 5GB or more from what i remember. Too bad they don't unify all their solutions and market it properly.