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.