I have a website on which I am currently running Google Adsense.

Looking ahead, if someone wants to buy advertising on my site, I would need an ad rotator or ad management system to be able to sell a certain number of impressions, right?

What are the most popular ad rotators that people use?

Are any of them free?

OpenX will do the work.
http://www.openx.org/ :)

You want to talk about the biggest pain in the ass to configure in the world, then use OpenX....

I can't remember off hand what the software is called, Ad Rotation something, but it's excellent to use.

It's not hard to configure, it's hard to understand for the first time you use, but it is well documented. :)