I made random encounter tables for various terrains (plains, forest, hills, mountains, moor, swamp, desert, coast, dungeon) - basically just a list of monster names - and I've written up a few pre-built encounters by level. I started a thread on it a week or so back but haven't built any more encounters.
My system for random encounters is: roll 1d6 once per 4 hours outdoors, on a 1 there is an encounter. If there is activity in that hex (from a nearby lair) then there's a 50% chance it will be with those creatures; otherwise, roll on the random table.
I can't remember exactly how it works in dungeons.