So before you slag me with flames with NO ADS IN SPIRAL KNIGHTS, I'm not talking about in-game.

I play Android games, and a company called Tapjoy has a currency for clicks thing. They sponsor certain games and apps, you DL and open them, and you get a small amount of in-game currency. There are also retail companies to use Tapjoy to give rebates and discounts, and usually there's a ton of currency involved.
For example, Valentines Day was looming, and FTD offered 55 Platinum (probably about 7 or 8 dollars worth of currency) to order thru tapjoy, and at a 30% discount.

The 1 platinum clicks, say for DLing demos, answering surveys, etc, can be for 10CE, whereas buying a game or program thru a sponsored click-thru site could garner you 10, 20, 30kce, depending on what you buy. (heh, gimme a clicky to Adobe pls. I'll be wanting CS6 and 200Kce for that grand I'll be paying)

Best of all, it's a passive system that allows you to either access a play store, then the tapjoy server, or a button on the login screen. No in-game ads.