I'm currently using AdSense, and am very happy with it. But, I do have some unfilled ad space that AdSense simply can't reach due to TOS limitations.
I've seen a fairly common ad type that is commonly shown near the top of the site and is very narrow (like 1000x30), but when you mouse over it or click "Open", a larger ad opens. I like this style, because it's non-intrusive, and since the ad is only shown to people that are interested then the CTR has to be great.
Can anyone suggest an ad network that utilizes this type of ad unit? My site has about 6 million monthly pageviews, so it's OK if the ad network is somewhat exclusive to higher traffic sites.