money
I've followed AJ's experiment in funding free software development with his AJ Market with interest and have thrown money in the pot once or twice for some of the projects I care about. It's an interesting approach.
I'm too lazy to do anything like that, or rather, I'm already paid sufficiently to work on free software that doing a joeyh market would too much resemble work. However, I did decide to try something I've seen done occasionally, since I also wonder how that works out -- I added a paypal donations button to the website for one piece of free software. Just as a test, to see if it annoys anyone, and if anyone bothers clicking on it.
I chose alien since it's well known, regularly used by a large number of people and even appears on the first page of otherwise quite amusing results on a google search for "alien". (I suppose that adding google ads to the page would generate more money, but in a probably less interesting way.)
Unlike AJ, I won't promise that clicking on the link will motivate me to work on anything. In fact, the most likely use of any money I get from this will be to pass the buck along to the AJ market or something similar.