I am a member of a number of webcam sites, but keep cumming back to I'm Live for the huge variety of models that are online at any given time. Even in some more of the obscure categories, like trannsexual, there are a huge number of models always online.
The Happy Hour feature really sets this site apart from some of the others, where you can watch a model play with herself for free and also cam-2-cam with her for free, provided you have Happy Hour credits, which you can earn for free on the site. This allows you to take it in turns directing (and participating in) the show for around 2 mins at a time - you take in turns with all the other guys who are watching (but of course, it is only the model that can see you).
For the real action though, you have to pay and buys some credits. The credits packages on ImLive are very well priced compared to the competition, so you are less likely to go broke getting your rocks off.
ImLive is well worth checking out and it is well worth creating a free profile, even if you never buy any credits. (Read the full review)