Recommend hosted Ad Manager for displaying ads on web sites

What would be a good hosted ad manager for displaying advertising on a site?

I've heard OpenX, but never used it.

klausbyskov mentioned AdSense but it is not the same sort of thing at all. When you use it, you're basically handing over the selling of your ad space to the bidding mechanism at Google. That's not necessarily a bad thing, but it's not for every site. That's because you may be selling some of your own ad space if you're a larger site, then you need a tool that can mix multiple sources of ads together.

I've known several people who used OpenX but had complaints about it. Some have recommended Google Ad Manager as an alternative but I've not had a chance to try it yet.

It was requested that I add specific OpenX complaints. Here are a selection of quotes (from a website that may not be named):

1. OpenX is slow as "insert something offensive here".
2. It's fairly OK for a stock installation, "but if you're going to want to do much custom modifications with it, especially on the recent versions, just don't BLEEPING bother".
3. Not a fan of hooks.

This is just a selection of the complaints. However, all of the complaints were predicated on the idea that you were hosting it yourself. The version where they host it may be quite different.