Cthulhu Nation

Cthulhu Nation is a Web-based RPG with a Cthulhu Mythos theme. It can be played for free, but this is essentially just a trial mode, and a subscription (inexpensive) is essentially necessary to really play the game.

Because the main activity is killing Mythos creatures by the hundreds, it is definitely "cowboy Cthulhu" at best, but it does have quests that bring in more of a narrative feel.