Thinkpads have already been mentioned but I also highly recommend the Dell Latitude, especially the 7490. They have fantastic keyboards, great battery life, and absolutely amazing displays (among the best 1920x1080 displays I have seen on a laptop). Plus they run many Linux distributions out of the box with minimal configuration.

If you live in the US or Europe I'd recommend the US/UK Dell Outlet. You can get a Dell Latitude in great condition (often like-new condition) for a nice deal, especially when they have their laptop promotions. I've had a good experience with their onsite service as well.