Since Gmail came out there has been a lot of controversy over the text-based ads based on your emails (though, granted, most of that has died down now). With all that said and done, I must personally say I find the Gmail ads overwhelming useful. They are, for the most part, well picked, and often provide useful information. I’ve now clicked quite a few of their links, and actually use a number of the things they’ve advertised.
Hail, context sensitive advertising. Welcome, and please bring with you relevant text ads to replace banners, popups and things that flash.