The coolest distro I've found in my travels was Crunchbang Linux...hands down. It is no longer in development, but the community still carries on in it's spirit with "Bunsen Labs Linux".
It is a minimalist openbox desktop based on debian. Openbox provides a snappy and highly customizable interface with almost nothing extra included for eye-candy by default (although there is lots of room to add your own)...just sleek...mostly black and grey themed...simple.
Out of anything I've tried, it is by far the most stable experience I've had in a Linux desktop with almost everything you could possibly need working right away after install. Also, because it is openbox, there is not much in the way of gui for configuration and instead provides easy access to text configuration files through the applications menu. I learned most of what I know about Linux under the hood because this distro forced me to. And the community was fantastic for support. The Crunchbang discussion board is still up and running afaik and remains a go-to source of reliable info for me.
I'd recommend it for anyone who is ready to move on from a large and easy to use distro like Ubuntu, but is not quite ready to dive into something as intensive as Arch. I've since moved on to building up an openbox desktop from scratch on my own from a minimal Debian install, but if anyone has not had this experience, definitely give Bunsen a test drive.