I wrote a maze-generation program [0] in 1999 that used disjoint-set forests with path compression to generate mazes. Wikipedia didn't exist yet, so I didn't know what they were called, but I'm pretty sure I'd heard of them somewhere before.
It's really more like 7 lines, with the bulk of the program logic in a single massive line that includes 3 comma-separated expressions and 6 conditional operators. It's called three lines because it's been mercilessly stripped to 79-character line widths.
That does however mean that it's a 237 character maze generator, which is still pretty cool.
[0]: http://lists.canonical.org/pipermail/kragen-hacks/1999-Febru... "mazewiz in C (second iteration), posted to kragen-hacks 1999-02-16"
Joe Allen's three-lines-of-C maze generator (included in the post), however, blows the hell out of my program for coolness.