Thinkpad X61. Solidly built. Compact. Linux runs out of the box. Currently running Fedora. I honestly never max out the 2 gb memory available - it can take 4gb, but I haven't had the need to upgrade yet. I did need to put an SSD in it.

It's been going for 10 years, and probably has at least another 10 years left.

Cons, does get a little hot on my lap from time to time. The little red joystick thing takes a little getting used to. I'm not much of a mouse user so it isn't too much of a problem, I run the i3 window manager. No space is wasted on a trackpad.