Comment If we're talking Specialist Distros... (Score 3, Informative) 221
Surely BackTrack needs a mention. One stop shop for Penetration Testing, Ethical Hacking, Security Analysis and pretty much anything else security-related. It might not qualify as a fully-blown "distro" depending on your definition, but it's a lot more customised than your standard "Clonebuntu" variants.
If you are even remotely interested in Network Security or Penetration Testing, it's a really invaluable tool.