Google’s AdSense has got to be the most painless way to put advertising on your website. There are no costs involved and a simple online signup procedure. The ads are text-only and relate to the content of the page. Google Weblog lets you try out AdSense with your site (or anyone else’s) without having to sign up.
Here’s Ads for microsoft.com, Ads for slashdot, Ads for bluebones.net and Ads for aol.com (which at the time of writing are all for pants, for some reason).
Google pays about 50 cent per clickthrough (NOT per view) and this number is falling all the time. Actual stats are hard to get hold of because Google is uncharacteristically reticent in publishing stats or allowing others to publish their stats (probably because Google is paying everyone wildly differing amounts). Personally, I prefer my site to stay free of ads. If I had more than the thousand or so hits a day that I am getting maybe greed would overcome me but as the minimum payout is 100 USD I’d probably be waiting months for that.