I use a Thinkpad. Not only are they some of the best laptops around, they also have great compatibility for Linux and are well regarded by Linux-users - so support for new Thinkpad hardware is regularly added. Still, you'd be advised to search around first just to be sure they're compatible. I'm using one of their cheapest machines, a Thinkpad Edge E520. It's a great machine with full compatibility with Linux (running Ubuntu 11.10 with Gnome 3). The build quality is the best I've ever had and the battery life is 15+ hours (of fairly heavy use, not just when idle). It only cost me £340 (including a battery upgrade) :-)