Yahoo has been very kind to me - I get free advertising for myself and friends. Everytime I catch them stealing bandwidth by means of hotlinking images off of my sites, I change the image but leave the file name intact so as to get the free advertising ... like this:
http://music.stage.music.yahoo.com/p...playlist_page/
You'll notice first off that the image is not Paris Hilton; and, if you right-click to and view the properties, you'll see that the image is coming from
http://www.isnowlegal.com - one of my websites.
But, back on topic ....
I use AdSense for publishing ads and making a few extra $$, but I have never done AdWords or PPC to promote my sites. Besides being frugal (a nice way of saying very limited budget), I merely take advantage of the same things Comdial mentioned: creating boards that link back to my sites and effective SEO that leads to good SERP.