Yes, you can set up for individual categories and advertisers can place ads on the appropriate page.

In AdPeeps you set up "zones" to display ads. Just set up zones like "top-23" 23 being the "type" in 68.

Then just add the Smarty call to the zone area in the AdPeeps code: &bzone=top-{$smarty.get.type}&

AdPeeps also lets you run rich media, flash, and text ads in formats styled after yahoo, google, or their own format. I run the teaser text at the top of my pages with it too.

It will handle any size ad, and you can even set a zone to handle variable ad sizes.

Not open source, though.

Payment through PayPal (supports subscriptions) or 2checkout. Price per click, per 1000 impressions, per day, per month.

I use the AdPeeps code in the templates, but it will also generate php if you would rather put it in a page that way. It's a free trial... totally. Check it out. I'll be glad to help you figure things out.