While the web sites mostly support themselves via the banner ads at the top of some pages, it would be nice to do better than "mostly". Also, I get a lot of folks saying they owe me a beer if we ever meet up; well, now they can buy me that beer with a couple mouse clicks. I've set up a button that lets you use PayPal to voluntarily donate $5 to ACME Labs. If you feel the urge to give, go ahead and click. PayPal will give you chance to change your mind, and then if you confirm it will do the transfer.
You can also set up a $1/month automatic subscription.
If you haven't signed up for PayPal yet, check it out. It lets you transfer money from a credit card or checking account to anyone with an email address. It's great for eBay auctions.