Open Source: TinyMe
If you are interested in getting that really old machine up and running again, with as little as ~32MB of ram, then there is a shiny new competitor to PuppyLinux and Damn Small Linux: TinyMe.
I know that Ubuntu is all the rage now, but I have had some serious issues with getting wireless working on my Thinkpad (Intel 3945 a/b/g) when using the roaming feature (e.g., at coffee shops, airports, etc.) and so have been using PCLinuxOS for the past couple of months; the fact that it allows you to use the ‘productivity’ of Beryl (think 3D spinning cube, Mac-like Expose feature) is only a plus.
TinyMe is a natural fit for such machines, and the speed and ease of installation, as well as the ease of use once it is setup are both huge pluses in its favor.