I've been using Portable Firefox since it came out, to be able to use a customized Firefox install on any Windows computer. I recently also installed Portable Thunderbird (email), Portable Sunbird (calendar), Portable Gaim (IM), and Portable FileZilla (FTP). I've also had XMPlay installed for awhile, for listening to almost any music format (the Shades skin is small enough to remain on top of other windows). I keep the UltraVNC client on there in case I need it.
PortableApps.com says it "is a community site devoted to the development, promotion and use of portable applications." They have a good sampling of apps to install on your flash drive or portable hard drive, and seem to be ready for growth. Maybe a good one to bookmark...