http://amanita-design.net/games/botanicula.html
It's truly wonderful - so incredibly imaginative and atmospheric with lovely graphics and sound/music - AND it makes you laugh. You'll have a fixed grin on your face when playing it, it's so incredibly entertaining.
It's available for the PC, Mac and Linux.
Dead cheap too if you go for the Humble Bundle option of paying whatever you feel like:
http://www.humblebundle.com/
Over 54,000 copies sold in only 3 days - can't be bad!
