Meetup: is this free?: "Yes, MEETUP is a free service. In the future, we'll have special features that some users will want to pay for. Also in the future, MEETUP may charge cafes (or other good places for a MEETUP) to be listed as potential MEETUP venue."

Meetup is "a local gathering of a group of people brought together by a common interest. Real world, face-to-face -- over coffee, over a beer maybe. MEETUPs are like book clubs, activist groups, shareholder meetings, user groups, fan clubs, car clubs, support groups, or study groups. All in local communities."

I expect an increasing number of sites to start for free but explain their users from the get-go that they might or will add paying services in the future. This one states it's not even going to try banner and pop-up advertising.