Even if you're not remotely considering adding Google AdSense ads to your site, here's a very fun tool -- you just pop in your URL and it displays the sorts of ads that would appear on your site.
In my case, I'm not adding AdSense to our sites, because in my niche (corporate execs) brand is a critical thing. A few lame ads targeting a slightly off demographic could make visitors think we're for people "not like them."
Or, maybe I'm just a control freak.
BTW: You could use this tool to scout out potential advertisers to sell directly to.....