Further to others mentioned above, I have to mention the Barn Runner series of games.
http://www.barnrunner.com/games.html or
http://www.bigbluecup.com/games.php?...rm=barn+runner on the AGS website.
Honestly some of the funniest games I have ever played (freeware or commercial), very well written, brilliant characters and game world and an interesting and really funny story.
The game also has different paths, with some decisions taken even affecting later games in the series.
Presently 5 games in to the series (together with some "spin off" "arcade" style games which aren't necessary for the main story but are fun in any case).
If you haven't played them then you need to do so. Now.
