Throw in your thoughts and recommendations here (brief explanations might be nice):
Ubuntu - is based on Debian and tailored to human needs (read : easy to use).
Debian - Is more secure than the other distros and it supports several architechtures:
-Alpha
-ARM
-HP PA-RISC
-Intel x86
-Intel IA-64
-Motorola 680x0
-MIPS
-MIPS (DEC)
-PowerPC
-IBM S/390
-SPARC
--OlssonE--
Knoppix Learn linux and leave windows intact.
~Rinswind~
Puppy Run from CD or install to USB, Zip, or Hard drive media.
Small, fast, and very newbie friendly.
N.B : as of August 2007, most Linux distros are available as "Live installation CDs", eg. you can try first from a bootable CD
and then install.