I like Five Across

Five Across - Workgroup Instant Messaging, IM, File Sharing, and Project Management

"InterComm™ is next-generation instant messaging software that makes communication more productive for team-based projects."

Tried it, liked it. Available for Mac OS X 10.2 or greater and Windows XP plus Windows 2000.

Five Across in some ways reminds me of HotDesk. I was completely sold on it in the late 1990's and strangely pleased to see that they are still around. A similar (and multilingual) web-based collaborative service is provided by Swedish Projektplatsen. I don't have any hands-on experience of it, but people I trust rave about it.