According to me there are four great affiliate networks where you can make a serious cash. I think that your first choice should be Linkshare because it has many premium merchants where you can make nice selling commissions.

The second one is Market Leverage with lots of best converting CPA offers.

My third favorite is RocketProfit which works also mainly in CPA offers.

And finally Commission Junction. It is harder to make money here but if you have a niche blog you can get nice commissions from several known advertisers.