One of my favorite early computer graphic modes is the ZX Spectrum's standard mode. The field of 256x192 pixels (roughly less working area than four Retina iPad icons) is broken up into discreet 'atribute blocks' of 8 by 8 pixels. Each block can use only two colors out of fifteen available. It's a kind of limitation that can result in a lot of interesting style choices. I especially like the highlights on the cathedral. I wish I knew a pixel art tool that enforced this mode so I could tinker with it without needing to fire up an emulator or purchase exotic hardware.
Picture from http://noirlac.tumblr.com