Other good ones to check out for collaboration and project management are Smartsheet at
www.smartsheet.com and Basecamp at
www.basecamphq.com/. I've used both of those as well as Clientspot. I liked Basecamp the best out of all of them for collaboration, tasking, files and messages. Clientspot is also good. Smartsheet take a bit more getting used to but does have the feature of just sending an email to the client for updates and feedback which eliminates the need for them to login into the system.
If you are just looking share files, you might try Box.net.