This link was on Slashdot this morning. It lists a dozen very neat Linux distributions, some known (such as DSL and Parted Magic), but most are very useful niche distributions. BTW, there is an add that loads first (and slowly - it's in Australia), but they give you a "skip this adv" button which works...

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