Been playing with Truchet tiles in Processing:
This is pretty random and messy but can be interesting. I plan on doing something a little cleaner and more tile-esque next.
Roughly speaking, a Truchet tile is a single tile that will match up with it's surrounding tiles regardless of it's position. For example, this is a very basic one, rotated through all four possible angles:
Some people have played with this concept to make some really amazing works, such as this on Flickr: