I like these useful distributions that have special purpose. I write some down below, do you have any tips about other useful distributions of this kind?

Backup and recovery:
http://www.clonezilla.org/

System rescue:
http://www.sysresccd.org/

Fix Boot:
http://www.supergrubdisk.org/

Firewall:
http://www.ipfire.org/

Network-Attached Storage (NAS):
http://www.freenas.org/

HTPC (more fun than useful maybe):
http://www.geexbox.org
http://xbmc.org

dixiedancer wrote:

For kids (little kids, I mean, pre-teens) there's [url=http://linux.softpedia.com/get/System/Operating-Systems/Linux-Distributions/Qimo-4-Kids-45702.shtml]Qimo4Kids. I put it on some old throw-away 'puters and donated them to a children's charity. They love it!

I have to try that one out sometime, that Laby Programming Game sounds interesting to test.