An FTP application is ideal to keep on a pocket drive, because then you can move files too large for e-mail-even long video files-to colleagues or a remote machine with ease. Cyberduck (see below) is a free, open-source application that can do either FTP or SFTP (SSH Secure File Transfer) transfers. Unlike a lot of FTP programs, it integrates with a number of Mac OS X's built-in features. For example, there is a Spotlight importer for bookmark files, and synchronizing remote directories with local ones will feel familiar to any regular Mac user.

I also like a quick-and-easy text editor on my pocket thumb drive—nothing fancy, just enough to let me write on the go. Someone recently suggested TextWrangler. It's a free download, and after working with it I'm impressed with it. It's especially good at opening remote FTP files. Note though, that this one isn't a miniature application. It has a 22MB footprint.
Instant messaging is often an ideal on-the-go way to communicate with colleagues, and a good thumb drive application to have for chatting from any Mac system is Portable iChat. It's basically a little script that will open a local copy of iChat, and include your usual buddies and preferences.

If you like to have all your browser preferences set up as usual on any Mac system you find yourself using, get Portable Safari on your USB thumb drive. It's a script that sets up your preferences and bookmarks on a local copy of Safari when you open it.
When you're on the go with a pocket USB thumb drive, the ability to quickly get something from your remote Mac system can be a lifesaver. For a free application with a tiny footprint (only 852K), get WakeOnLan. You can even schedule it to turn on your remote computer and do scheduled backups.
Do you know of any essential portable Mac apps?