This is a very helpful page, carefully picking apart differences and similarities between distros.
I've heard that Damn Small Linux is very useful for limited space. It is a live cd.
Wikipedia on Damn Small:
"It can also be installed on storage media with small capacities, like bootable business card, 64 MB USB flash drive, CompactFlash card, and Zip drive."
Damn Small "originally started as an experiment to see how much software could fit in 50 MB"
"demonstrated browsing the web with Dillo, running simple games and playing music on systems with a 486 processor and 16 MB of RAM"