I played the free trial of World of Warcraft, and I would have signed up if I hadn't already had a subscription to City of Heroes.

The Warcraft play seemed limited only by a 10-day time period. Even with a day job and 2 hour roundtrip commute, 10 days was plenty of time for me to get the feel of the game and want to sign up. One feature of the trial period was that at the end of it, you were able to sign up and keep your character at its current state. You didn't have to create a new character or start back at L1 once you signed on.