A few years ago I had a site that was generating about 4k unique users per month for my Linux/FOSS software program. I thought I'd throw on the Google AdSence. After three months the AdSense control panel said I accumulated $2.53! It wasn't long after that when I got a nasty email from Google saying that due to "unauthentic" click activity, my site account was disabled and I was permanently placed on the AdSense shit list.
I later found out why. Some enterprising user on some forum thought he would support my work by clicking the Google provided ads repeatedly. Following Google's appeal process, I tried explaining this by submitting my story through their online form. This was 2008. To date no response, and I'm still on the shit list.
I later found out that the best way to make money with online Linux/FOSS software is to use PayPal's donate button allow users to contribute to my "beer fund". While PayPal's fees were atrocious, I was able to buy a case of Guinness. YESss