Free ones are few and far between and in my experience you get a lot of, how shall I put this, errrr......not very nice people! I have never used one by the way, my friend used to go on them.
Plentyoffish and Badoo are both free. Be warned though - friends who use Badoo tell me that a lot (read most ?) of the men on there are looking for sex and that a number of the "single" ones aren't. Don't know what the POF site is like.