www.eharmony.com advertises themselves as having more serious about dating members, primarily because of the pretty lengthy profile that you are asked to fill out. It's also a little more costly than some of the other sites.
I've heard good things about them from other people who have had success with it (all men) my own experience with it, was it's the same assholes that prowl every other site (which pretty much leads me to beleive I give really bad ad...
I don't know if they have a trial period, but it's definitely worth checking out. The guys I know who have used it, claim they've met women of a little less skanky than some of the free dating sites.