I use a thinkpad t61p. I've had it since 2011 and it still runs just a good as it did when I bought it. Its extremely modular, rugged, and has fantastic linux support. The only thing that I don't like about it is that it's battery life is pretty sub-par but most places that I use it have power outlets so it hasn't been an issue yet. It also has the best keyboard that I have ever used in my life by far.