I've just added a Google Adsense bar to my blog in the hope that I'll make a little cash. The minimum required before Google will actually send you any money is $100 - how many clickthru's that equates to I'm not sure.
If you're interested in adding it to your own blog/web site it's quite easy. Just go to https://www.google.com/adsense/ and apply for an account. Once you're approved (took a couple of days for me) you can login and choose the size/shape/color scheme of your ads under the Ad Settings menu. When you're happy just copy the code they supply at the bottom of the page and paste it into your site.
If you're a Blog-City user here's some instructions for putting it into your gutter:- go to the PORTLETS menu of your admin and click the 'manage' link under Custom Portlets. Give your custom portlet a title - e.g. "Google Adsense" - and paste the adsense code into the body (note you'll need to be in html mode if using the advanced editor). Save that and then it's just a case of choosing where in the gutter you'd like to position it - Google recommends you stick it "above the fold" (i.e. in the area readers will see on first loading the page before they have to scroll down).