Assume you mean REASONABLE or cheap advertising?
Adbrite and Blogads are worth checking out. Stumbleupon has really cheap rates of 5 cents per visitor and they deliver real visitors. I tried a small campaign but could not tell how good the traffic was. I've heard others say their traffic doesn't convert very well. But guess it depends partly on your site and how well you can target their traffic.
As with most things in life, you get what you pay for.
