A great option I recommend is in game ads. I usually put them on the preloader I've found they are not intrusive here and once the game starts there is no in game ads to annoy people. I'm not a fan of inter level ads, I don't like to ruin games as I've seen others do by annoying the player. Though some games are an exception when they have put the ad to the side of an inventory and it isn't in the way. Smart design is the key here.
My favourite is probably CPMstar, they offer an easy API and quick approval by a sponsor. The catch is you have to share 50% revenue with your sponsor but that is very fair if you make a good deal with them and it helps your sponsor trust you more for future sponsorships. So ads can benefit the developer and the sponsor as well.
With CPMstar it really varies on how well a game is doing. I usually make about $10-20 a month off them on average with 4 games using CPMstar. Sometimes I've gotten $50 when a game did well like Heli vs Tower. I figure the more games I release the higher this will get per month.
Mochi offer higher click through rates then cpmstar however you can only withdraw your money when it reaches $100. Mines been stuck at $50 for ages, it's frustating, but if you have more games with it can be good, specially if you're starting out and don't have a backing sponsor. They are overall good but still needs improving.
Newgrounds offers there own in game ads now. It's not as good as the other two above though. The click through rate is a lot lower and the ad sometimes doesn't display anything but the NG logo. However they are a good quick option with easy API if you want to support a side version of ads with a website that isn't fully based on it. Plus it adds up with your in website revenue. I released one game with it as a test, the game didn't do well, it's made about $100 from these ads in a few years.