Edit: 2010~2011 era Thinkpads actually make great cheap Linux dev machines. If you want something a little smaller than the 15" T520, the T420 is great too. And if you want something "ultraportable" the X220 is a great choice and can be had for under $100 used on Ebay.
Many companies tend to replace them after a few years so there are a lot of them on the market, and they are generally quite cheap.
Once thing to keep in mind however is the screen. At least for older ones, it tends to be bellow average (low resolution, low contrast, low fidelity colors, etc) as companies tend to "forget" to check the "good screen" option. It's obviously a generalization, there are some with good screens, but keep it in mind.
I'm the happy owner of an x61, upgraded with 4Go of RAM and an SSD, plus a new battery, it still going strong.
Personally, I'm always reluctant to pay a lot for a laptop. A laptop can, easily be stolen, fall one the floor, be damaged in any way. Cheap secondhand thinkpads fit my needs wonderfully in that regard. A few years ago, just 2 weeks after I bought my x61, I slipped on an ice patch, fall on it heavily and the screen was dead. I just bought another one, exactly the same, swapped the disk and I was good. It wasn't a big dent on my budget, it would have been a different story with a brand new Macbook Pro.
I've a thinkpad T460s (i7) at work, it tends to overheat a little, I would not recommend it. And there are a bunch of X1 carbons there to, they are not very reliable machines. Personally I would stick with the basic X2x0 or T4x0 models.
Thinkpads are really easy to repair, easy to disassemble and there are a lot spare parts on ebay (keyboards, battery...)
I couldn't find it again, but back in the day, there was a webpage which gives you the configuration details from the type number at the bottom, it was really convenient to get all the config beforehand from the seller, you just needed to ask in this type number. Don't know if it still exists.