I was browsing DistroWatch and was looking for a list of distributions that offer rolling releases and a little bit more about them. However, it doesn't seem to be possible to select only the rolling releases on DistroWatch. Perhaps the Linux community can offer some insight into what distributions are available that offer rolling releases.
I've made this question a community wiki with the hopes that I'll get one distribution per answer with a brief description of the distro and what it offers and a link to the distribution's home page and/or DistroWatch entry. Then, people can vote up/down the distributions to rate them in terms of something (best/worst, pure popularity).
(Also, if someone can clean up the tags on this, that would be great. I wanted to create the "rolling-release" tag, but can't.)