Quick and Dirty Games makes some wonderful free games.  Thrilling Noir Stories is one I came across recently.  At only eight pages, it is easy for a player or GM to browse through and get started.  As a Quick and Dirty Games game, Thrilling Noir Stories requires some adjustment by the GM.  While not unusual for most games, the rules-light nature of the game is quite appealing to me, especially for this genre.  Noir is a genre near and dear to my heart, and the game’s rules manage to capture the tone quite well.  Background reading is definitely required, since the PDF focuses entirely on rules and leaves setting, characters, and adventures up to the players.

Recommended reading definitely includes works by Dashiell Hammett and similar writers.  There are also plenty of noir films available to bolster one’s understanding of the genre.  I’d even venture to suggest the Dresden Files as a good series to examine, even if it is a largely supernatural series.  The rules are quick to learn and definitely make sense.  I haven’t had a chance to playtest it myself, but a thorough reading of the rules leaves me feeling quite optimistic.  I don’t know that it is necessarily any better than an adaptation of a generic system, like Savage Worlds or GURPS, but it does use a d20 system and so will be somewhat familiar to D&D devotees.