I have had great luck with StumbleUpon advertising, but it really depends on the site you're trying to promote. The problem with SU is that you need to capture

I have had great luck with StumbleUpon advertising, but it really depends on the site you're trying to promote. The problem with SU is that you need to capture the user in the first few seconds, or they're just going to click "stumble" and bounce to the next site. But if you do have a fun/interesting/cool site, SU is a great source for consistent and cheap traffic ($0.05 per visitor).