Voodo Labs Sparkle Drive (very cool pedal)
Morley diamond distortion (out of production, but you can find 'em on ebay)
Boss od-1 (a standard)
Tube Screamer (I find it lacking in the lows, but you can re-eq for that)
Route 66 american overdrive (very cool)
TRex Mudhoney OD (expensive, but very cool)
Fulltone (anything they make is KILLER)
actually, one of my favorite od pedals is "FrontLine Series II Overdrive". It was made in England in the 80's, I got it for 20 bucks at a pawn shop, never seen or heard of them before or since, didn't listen to it before I got it,(but for 20 bucks who cares, y'know?) and was pleasantly suprised with the natural response of the unit.
I recommend going AGAINST the mainstream, as everybody and there mom has a tube screamer. But if you want to sound like everybody else, I guess a ts808 is the way to go.
To many good choices....go to a shop and use your ears! Happy hunting! I love getting new od pedals!