This list includes distros for specific tasks:

Damn Vulnerable Linux

Tinfoil Hat Linux

CAINE (digital forensics)

CAELinux (engineering)

a gaming linux

Parated Magic (for manipulating your hard disk)

Musix GNU + Linux (for making music)

ZeroShell (for embedded systems)

Mythbuntu

and some very very small distro’s.

12 of the most interesting, unusual and useful Linux distros – operating systems, open source, Linux – Good Gear Guide.