A list, I am compiling with your help, of games that have free trial periods.
The longest I am aware of is my own favorite, Ryzom, which has a 21 day trial period. Yes, I am opening plugging this game.
Other lists I have found you can browse in no particular order
MMOhut has a list of free (completely free) games here
And a list of free trials here
and here’s another set, but this is a top ten list