My favourite is
Damn Small Linux.
It's debian based (tho moderately modified), runs lightning quick (even on old hardware), can run from a CDR, USB drive, HDD or within another OS (e.g. Windows), and they've devised some truly clever systems for adding apps.
I run it at home on my 100MHz/64Mb tinker box, using a 256Mb CF card and CF-IDE adapter.