In today's internet market, I doubt you'll find a truly free one. If you don't pay with money, you end up "paying" with very annoying pop-up advertising.
I have been using ezboard for over a year now for some family discussions and games. Last year when I set it up it was completely free (no advertising). But with the way things have been going economically for such companies recently, they have started with really annoying advertising. So a couple months ago I paid...but I got a few people in my family who use the board quite a bit to help split the cost. It's pretty affordable actually.
"Free" went away when people realized you can't stay in business that way. Duh!
Yahoo Groups is not too bad. Been using it for over a year for two different car groups, one with 80+ members, the other with about 70. Works good, except that you have to deal with the occasional advertisement.