A lightweight derivative of Ubuntu Linux, CrunchBang is designed for the limited display resolution (and the hardware profile) of Asus' Eee PC range of netbooks. It's also limited in size, offering a few basic apps like Firefox, but otherwise maintaining a minimal footprint: Just slap it on a flash drive or CD and get cracking. Here's a forum thread about it.
As much as Linux confuses and disappoints mainstream users, projects like this are a much better path than the current trend in netbook software, which is "Put as much RAM in as possible and hope Vista won't suck too badly."
It's also perfectly named for Mike Arrington's tablet netbook thingy, if and when it appears! (And also comes in a standard edition if you want to put it on anything else)