At my current job we use FogBugz and it's nice. I don't really see anything too special about it. I only use the basic features, but other people seem to get really into it and some of the features it provides (such as the email integration).

I've previously used Unfuddle and found it easy and intuitive. I liked it a lot. I've also met the two guys who started it a couple times. They are based in Honolulu, it is in Ruby/Rails, and they host at Rackspace. They seem to be doing very well.

I've looked, but never used a few of the others like Github, Bitbucket, google code, and Trac.