I'm in the same boat as you, I run Linux, my girlfriend OSX and friends/parents have windows boxes. I started with portable firefox for both OSX and windows. Same for putty, ftp client, etc... But in the end I realised all I had on there was for online use. So instead of using all those programs, I just put portable firefox on it and added some extentions, like fireftp for ftp and sites like google notebook, meebo. Now all the other things that are on there are a couple of files (mp3, txt snippets), firefox portable and qemu with a basic linux install.