Google Adsense is the current leader in contextual advertising to web publishers/marketer. I started using adsense since last year and earned decent amount from it, though I'm still looking for alternative which gives a higher CPC based on my niche.
I have an arcade site which is averaging 140 clicks per day and only earning 6$ to 8$ per day. Every arcade owner know how low is CPC(Cost Per Click) produces by arcade site. So I have decided to try widgetbucks which has a good feedback from a friend(Macuha).
Widgetbucks is a PPC based model and has a contextual algorithm automatically targets the right products for your site same as Google adsense. Their ads look great and easy to customize.
I just joined yesterday and already earned $11 today. I have not maximize the use of widgetbucks, that earning is only coming from 300x250 and 160x600(alternate with adsense) banner ads.
UPDATE: DONT'S SIGN UP. WIDGETBUCKS IS A BIG SCAM
They are giving $25 sign-up bonus which gets you half way to your first check!. Sign-up here, grab this opportunity and start earning!. By the way, they pay via Paypal if you don't want to send your earning through Checks.