Airliners.net is a free site. The paid "First Class" membership eliminates ads.
Ah. Was this a change in the last year or three? I seem to recall not joining it a couple few years ago because it was paid. I'm pretty sure this was formerly the case, because I remember being interested in some topics there and wanting to post but not being able to.
Cecil Adams' The Straight Dope message forums were formerly paid-only but are now free (and he wrote an article about the decision to do that recently), so it seems that other paid forums are moving to open, free access, too, so there would be precedent for a.net to formerly have been a paid site.