How about just affiliate programs?
Ones that have people sign up, then you get paid.

Ebay Affiliates is a high paying one.

I don't know of any networks that do what you're requiring because I can't see how it would be beneficial to them. Why take the risk giving you a flat monthly fee if the traffic you are giving them doesn't convert or provide some sort of ROI for the advertiser.

There are networks out there which pay on a CPM basis - which basically means that they pay you per ad view instead of ad click. A couple of these include tribalfusion.com, fastclick.com, valueclick.com ... ValueClick actually recently just bought out FastClick but they're run as independent networks for the time being. Relatively easy to get accepted into, as well.

Try Paid link ads, adbrite, linkadage or linkworth. There are plenty others out there.

hexstar, have you ever tried selling ad space yourself? You might be able to get a few bites if you advertise a flat monthly rate, and the best part about it is you don't have to share the revenue with an agency.

csgal,
Have you checked TextLinkAds and LinkWorth they sell pure text ads on your site for a monthly price.

I currently sell text links myself.

Who do you use to publish your text link ads?

Try exactseek.com. You can purchase three month packages, edit your ads, even change keywords if the alternative is available.

You can also try widgetbucks.com .

Note: I am not related to the website in anyway.