The only online job site I have used is Monster.com. I currently have my resume in their database, and I have had a handful of requests come from it. I had one person who had a serious offer extended to me because of what he found on the site, but I had to tell him I was not actively looking. I keep my resume on their site for 2 reasons: to keep my options open and as a safety net should something happen to my current employment situation.
I have always been a fan of careerbuilder.com. My resume posted on this site is actually how my current employer found me. I have received upwards of 45 legit job offers over the past few years from the resume that I keep with them. I give it two thumbs and two big toes all up.
I've used both of those mentioned, monster.com and careerbuilder.com, and in California there is a website maintained by the State at caljobs.ca.gov -- this last one is a great resource, but can be slow to update information, and it doesn't work with Firefox...
In addition, there is craigslist.com, which has grown from a dinky little info site to a huge international phenomenon!