There is this freee heatmap software that shows where users hover with their mouse or click. This can help you with ad placement. Problogger is far more eloquent than me, so check it out on his blog post here: http://www.problogger.net/archives/2007/08/16/create-a-heatmap-of-where-readers-click-on-your-blog/ I hope this was helpful!
Yeah, it's free. I'm using the free version which allows only data from 6000 site viewers, but that's all I need. Even Problogger says all over the post how you get it for free.