For some time now, I have been using Google adsense for showing ads on this site. There are a number of players in this domain. Shareresults is an interesting service which allows users to refer sites. For advertisers, it allows a number of methods in which a user may refer a site -- from banner ads to simple text ads. Payment modes are also very convenient. You can easily switch between paypal and check. A quick look showed me a number of sites which I may like to promote!