I want to share with you one of my secrects (well, tons of people know about it, actually) that is by far one of the best collabroation tools on the Internet today: TitanPad.com
It's super-simple to use, super-free to get, and you even get to choose your own color, which is also super. It offers file uploading, real-time editing, instant messaging and so much more. We're talking about top-of-the-line collaboration.
I've been using TitanPad for several projects now and enjoy it every time. The one downside is that it's a little funky when it comes to adding someone to projects. You'd think it would be as simple as using the project's "Invite" button, but you first have to add the user on the back end. But, once you figure that out (which I just did for you), it's really not a big deal. I'd also like to see more colors, or a color wheel.
TitanPad is my first choice when it comes to collaboration. If it does start to charge a fee, I really hope it's not a lot. Ten dollars, tops, a month for all the features would be a great price and totally worth it. On a similar note, I wouldn't mind seeing TitanPad sell a custom server option where customers could buy the code to place on their own servers, like an SSL server or maybe a LAN.
Good job, TitanPad.com!