Trail of Cthulhu
http://site.pelgranepress.com/index.php/category/products/trail-of-cthulhu/
Trail of Cthulhu was my first exposure to the GUMSHOE system. The premise is simple: clues that move the story are always be available to the characters. It answers the question of "I roll for [important clue] and failed. Now what?" Instead, clues are available but they might have major cost, or your characters may need to do more digging. It also adds stability to the sanity mechanic so the feeling of decay in your character is palpable.