An Act of Murder is a classic country-house mystery: an isolated estate, a small group of suspects, a limited amount of time to solve the crime.
The country-house mystery premise has been done numerous times in interactive fiction: consider Infocom's Deadline, or Sierra's Mystery House. Even in the best of these, though, the game-play almost always fails to capture what's essential about detective fiction. Even at their best, mystery IF games usually test the player's thoroughness (have you looked under every bed? analyzed every object?) and patience in replaying (have you tried spying on all the characters at all the available times of day?), rather than his logical thinking and deductive abilities.
An Act of Murder stands out because it does ask the player to focus on drawing conclusions: what do you know? What do you need to find out next? What do these alibis mean? There are a few objects to discover, a few pieces of evidence that have to be searched for, but for the most part, Act of Murder is about the conclusions you draw, and how you figure out where to look next.
The gameplay is also smooth and friendly. Note-taking is automated, so everything you learn is jotted down for you in your blotter. There is a time limit, but it is quite generous. Hints are built in. The fiddly aspects of solving the murder are taken care of by the game, so the player can focus on the logical puzzle of resolving who did it. At the end of the game, you win by submitting your collected evidence to another policeman, who comments on the significance of your discoveries; if you have any remaining doubts about what your evidence means or whether you're arresting the right person, this sequence lays them to rest. There's little chance of winning the game by accident or reaching an ending without fully understanding the plot.
The other stand-out feature is the game's replayability. When you restart, An Act of Murder assigns a new murderer, motive, weapon, and set of alibis at random. One might expect that to be clunky or to get predictable fast, but in fact it works very smoothly: I played five times and found that each time the plot was sensible, well-constructed, and consistent. After you play enough times, the mechanisms do become a bit obvious -- but Act of Murder provides enough substance to make several run-throughs fully rewarding and enjoyable.
Note that, like many freeware interactive fiction games, An Act of Murder comes as a downloadable file that can then be played on a platform-specific interpreter: interpreters for common systems are listed above.
An Act of Murder took second place in rec.arts.int-fiction's Interactive Fiction Competition 2007. The other highly-ranked games are worth a look as well.