Call of Cthulhu is the classic Lovecraft inspired RPG from Chaosium. It uses a very simple percentile based skill system. CoC also had a d20 version some years ago, but it is now out of print.
More recently, Pelgrane Press has put out Trail of Cthulhu, a RPG with a completely different system, centered around investigations.
Besides this specific games, there's quite a bit of Lovecraft influence in the RPG world... for example, the Far Realm in D&D and the Star Pact for warlocks in D&D 4e certainly owe a lot to his writings.
I know many of the folks at Paizo are influence by his work, and there is the Star Pact in 4e which is awesome. I have played the percentile CoC game, and enjoyed it quite a bit.
I was just wondering what sort of modules are influence by his work, or give an overall Lovecraftian feel of cosmic horror.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.