I'd like to emphasize how well Dropbox works as a collaborative tool. Never having to attach and download shared files from email is a great help in staying organized.

A co-worker can simply shoot you an IM or shout across the office that a document has been updated, and it's already on your machine ready to be opened.

I hope people don't get this confused with a source code repository though, as it definitely isn't optimal for working on shared source code. Stick with documents, images, and media, and you'll be fine.