Anchorhead

From wikipedia:

"It is loosely based on the Cthulhu mythos. Anchorhead takes place in a New England town by the same name that bears a resemblance to Innsmouth, Arkham, and other fictional towns created by H.P. Lovecraft."

and

"Anchorhead was hailed by critics and players as one of the best interactive fiction games available due to its complex and intricate backstory, well-written dialogue and descriptions, and absence of bugs (or even spelling errors). It won a number of awards after its release, including the 1998 XYZZY Award for Best Setting (it was a finalist for virtually every other Xyzzy award that year). As of October 6, 2007, Anchorhead is the highest rated game with more than one vote at "Interactive Fiction Ratings"."

The demo version for Anchorhead: Special Edition (to be released in November 2007) is not a complete game, however, the original .z8 version is provided.