One word, RaspberryPi. [0]
Though I use a small laptop as a client and sometimes at the machine with monitor and keyboard, most work I do is on a Pi, remotely. I'll be upgrading my Pi to version 2 and splash out the necessary $40.
Forty dollars, think about that.
All I need to hack with: #raspberrypi & #robertsradio ~ https://flic.kr/p/o1yXKe
FTR I support your decision to include your own constraints. I believe better things are developed from constrained systems than from without.
EeePC with win.
Now I could fire up the beastly server I have on my desk with 300W power supply, multiple HD's with linux, but as an experiment I moved all the dev over to a Pi. Add a few more Pi machines and I can use one for image processing, another for dev, another for sound/media & I have the core of what I used to do on a desktop for the price of a fast HD.