An interesting thing not many people remember is that in theory, every drow who reaches 6th-level may be tested by Lolth and immediately transformed into a drider upon failing. It's unlikely to come up in a typical campaign, but those fancy drow-only games everyone seems to be running now may see it occur.
Anyway: driders. 6 aberration HD, which are arguably equal to cleric HD (though with worse skills). This fits nicely with the 6th-level cleric/sorcerer/wizard casting they get: a feature that automatically forces them into LA territory. Cleric driders getting to pick from only four domains is a bit annoying, but not terribly so.
Driders are Large, but lack reach. They've got a single natural attack: a bite that deals piddly damage but injects strength-damaging poison. Their ability scores are quite high, with +4's and +6's across the board. Natural armor is pretty good as well, and a climb speed makes up for the lackluster land speed.
There's also some SLA's ranging from very situational (Detect Law?) to quite useful (Dispel Magic, Suggestion). SR and skill bonuses round it all off.
I'd say that a LA of +2 is, as many above me already noted, appropriate. Driders lose very little compared to full casters (which aren't exactly weak to begin with), and the goodies they get in addition balance out the lost spell level.