The grid of 9 alignments, more or less. However, except for powerful outsiders, undead or aberrations, it doesn't really figure into play.
For PCs, I prefer to just have a basic idea of character motivation in place to guide RP.
PCs belonging to an order of some sort (eg, paladins, druids, guild thieves, knights, samurai, etc) are expected to abide by their respective codes of conduct. I find this is a lot easier to work with than nebulous "alignments".