I just noticed a post by Seth Godin riffing that Ads are the new online tip jar. He says that "If you like what you're reading, click an ad to say thanks." which resonates quite well with me because this is what I've started doing over the last few months.
It doesn't take much to click an ad, wait for a page to load, then use the browser back button and get back to what you were doing. Takes a few seconds, and give the site creator something small in return. It's an easier system than Sourceforge's donation mechanism, or the many forms of micro-payments (which just never seem to take off) so for now, clicking ads seems to be the best option.
Note: Whilst I do have ads on this site I don't want you to click them just because you read this post. I'm not trying to drum up revenue.