Ad revenue generated from clicks by social users / multiplayer gamers is very lucrative.
Online charades:
http://www.websiteoutlook.com/www.isketch.net
Daily Ads Revenue $71.48
Brand new online arcade emulator:
http://www.websiteoutlook.com/www.supercade.net
Daily Ads Revenue $5.51 (this number will most likely increase)
Let's say for example that you had fully functional netplay in Retrocopy. And then let's say you had a webpage where users could see all currently hosted games. The user would click the game on the webpage, and Retrocopy would instantly loadup and connect both users to the game. So there'd be a lot of clicking going on within this webpage (gamers entering and leaving games all the time). And then what you could do is place a column on the right of the page, with an advertisment. People would be in the habit of clicking things upon this webpage constantly, so occassionally, you could get people clicking the ads. This is something that youtube does very successfully to generate income.
Cycle accurate emulation is nice and all, but multiplayer gaming and social activity is what is extremely popular. People don't want to play games on their own in a dark room anymore. People want to play with friends. We are living in the social age; facebook, myspace, mobile phones, anything to do with chatting and socialising is very popular and therefore very profitable. Let users have a profile page with age, sex, location, favourite games, favourite game company, favourite console, all the stuff that gets people chatting. Then integrate multiplayer gaming into the website and have ads.
The more you integrate all of this into a seamless webpage for ease of access (people feel less inclined to load up programs), the more popular it will be. Kaillera is practically dead now, thanks to the likes of supercade.net and GGPO.net. These services provide multiplayer gaming, but for arcade emulation only. The authors of these programs can't develop emulators, because they don't have the knowledge for that. They're just using the sourcecode from an emulator called "Finalburn" to host these services. The advantage you have, however, is that you have the knowledge to add as many gaming systems as you like, thereby providing a potentially huge array of multiplayer games.
Something like Unity 3d could even be used, to integrate Retrocopy completely into the user's web browser. (If you haven't heard, unity3d is currently the most popular 3d engine within a web browser):
http://marunchak.co.uk/unity/terrain/