Large advertising network paying around $0.15 CPM (here and below indicated efective CPM) for skyscrapers and banners. Applies $0.10 default fee (publishers have to pay when the network fails to sell advertising for your site) to sites that give them a large % of international inventory.

The banners are tied to popups. Requires minimum of 500K impressions/month.

Adtegrity

Pays an average CPM of $0.15 for above the sroll banner, $0.35 for banner / uncapped pop-under combination, and $0.75 CPM on popups. CPM rates fluctuate daily based on campaigns in rotation.

Contract is non-exclusive. Minimum traffic of 500,000 page views per month. 90%+ US demographic/target audience.
Pays net 90.
Good choice for your default ads.

Advertising.com

A mix of pay per click and pay per impression models. Poor reporting - confuses CPA and CPM ads so the CPM stats are meaningless. Usually CPM no more than $0.10.

Minimum payout is $25.00.
Your site must receive over 250,000 impressions per month and use English or Spanish as the primary language. One positive thing, that Advertising.com supports variety of below the scroll ads.

Burst Media

One of the best CPM networks. Rates are based on the level of targeting and range from $0.15 to $0.35 CPM for top 468x60 banner. They also have combined banner/pop-up campaigns that pay much more. You have option to send default impressions to another CPM or CPC network.

Burst supports banners, buttons, cubes, skyscrapers, and optional popup and popunder campaigns. But according to their strange logic you are not allowed to run any of their other ad sizes unless you're also running the top 468x60 banner at the same time.

Due to unsold inventory, currently only small number of sites are accepted. Does not accept search engines and directories, and prefers original written content. Payment is issued every quarter once commissions reach $50.
Recommended.

ClickXchange

About 5,000 of CPA, CPL, CPC and CPM offers. Many of their advertisers are guys with several bucks, so be wary.
Checks are paid monthly with no minimum.
5% commission on the earnings of every webmaster you refer, plus $10.00 for each referred advertiser.

Compact Banner

Rates vary from $0.15 to $0.50 per 1000 ads with "an acceptable click-through rate".
Currently pays $.41/cpm for banners with one pop up and $.15/cpm for banners without any pop ups. Count just unique visitors from US.

7% for 1st level referred webmasters, 2% - 2nd level, 1% - 3-6 levels. Minimum payout is $30.00.
Your website needs to have at least 5000 visitors per month. Also you can make money only as an affiliate.

Fastclick

Top quality network paying 65% of the revenues they receive from their advertisers. Huge $2 CPM for popunders (only 1 per 12 hours) and about $0.25 for skyscrapers and banners.

Does not sell site specific advertising, so for some "serious" sites this network is not a good choice. As a webmaster, you can choose which ad campaigns run on your site. Ads can not be placed in pages with discussion boards or chat rooms.

The program is two-tier, allowing you to refer other webmasters to the program and earn a 5% commission on whatever revenues they generate.

Payment is issued 21 days after the month once commissions reach $50.
Sites must be in English and receive minimum 3000 impressions per month.
Recommended.