Dude...Damn Small Linux!!!
http://www.damnsmalllinux.org/
You can always download and install Beryl to make a custom installation on your pen-drive!
Here's a few features from the website:
DSL is a very versatile 50MB mini desktop oriented Linux distribution.
Damn Small is small enough and smart enough to do the following things:
* Boot from a business card CD as a live linux distribution (LiveCD)
* Boot from a USB pen drive
* Boot from within a host operating system (that's right, it can run *inside* Windows)
* Run very nicely from an IDE Compact Flash drive via a method we call "frugal install"
* Transform into a Debian OS with a traditional hard drive install
* Run light enough to power a 486DX with 16MB of Ram
* Run fully in RAM with as little as 128MB (you will be amazed at how fast your computer can be!)
* Modularly grow -- DSL is highly extendable without the need to customize