Depending on the size of your blog network,
http://www.openx.org/ is the old phpadsnew which will do what you need and quite a bit more.
There are other software packages that will do ads, but, phpadsnew is pretty well known (though complex at times). Cleo wrote an article on the board a while back, but, I can't find it offhand. Even though it is for an older version, the technique is still the same.