PayPal – Make a donation. I had one set up on a site with minimal traffic and get a little every week. I gave options $1, $5, $12,000 (for a new car), and enter your own amount. A lot of 1′s and 5′s. No 12,000 – but I tried.
Bigger site with better traffic and make it funny, you might get a surprise.
Also like linking to Amazon and similar sites.