If you are doing something that doesn't require a fast machine (I.e. server side web development), then a late 90's/early 2000's laptop will do. I currently cart around a '95 Toshiba satellite pro 440cdt to use in low security areas, with the HDD swapped for a 2gb CF card. Linux, vim, nginx, several backend stacks. I wouldn't care if it broke or was stolen (like anyone would) - I can get another for £20.
If you need a machine for client side dev, then something from '05-'10 would probably cut the mustard.