The Kingdom of Loathing is a truly enjoyable, webbased, (kind of) black and white, funny, free, entertaining, odd and comical RPG. You can play as such things as an Accordion Thief, a Turtle Tamer or a Pastamancer. It also limits the amount of adventuring you can do each day, so you won't end up spending all your time playing it, although you're going to want to.

And best of all: This entry is not a shameless plug. I'm, in fact, in no way affiliated with The Kingdom of Loathing. I only like to play it.