PPC - pay-per-click.

Pay-per-click advertising (PPC) allows a business to display their advertising within search engine results or on other websites by bidding on one or more search terms and then to only pay for this when a user clicks the advert and visits their website. 

So, here's the list:

AdBrite

Description: Allows publishers to sell text ads on their sites

AdEngage

Description: Allows you to sell text ads on your blog.

AdGenta

Description: A kind of manual Adsense

Adsense

Description: The ubiquitous contextual ad program from Google

Affiliate Sensor

Description: Contextual affiliate text links

All feeds

Description: Varied offerings from contextual ads to search ads and affiliate links

Banner Boxes

Description: Contextual looking Adsense like ads

BidClix

Description: Allows publishers to sell text ads on a PPC basis

BidVertiser

Description: Adsense looking ads where advertisers bid for placement by site

Blog Ads

Description: Banners in all shapes and sizes where publishers join groups of similar publishers. Blog specific.

CB Pro Sense

Description: Provides auto generation of ClickBank affiliate links
Warning: Snake oil sale one page sales letter with lot's of exclamation marks and outrageous promises of instant wealth.

Click Ads Direct

Description: Unclear what the exact model is, but they appear to be pitching the exclusivity angle

Chitika eMiniMalls

Description: Contextually served interactive ads filled by Shopping.com and others. Allows readers to find reviews and more info on products aswell as purchase

Clicksor

Description: Contextual network matching to affiliate programs
Warning: These guys do pop-up/unders

Context Web

Description: Adsense look-a-like contextual ads

Crisp Ads

Description: Blog ad network where publishers can join the "catalogue" and have advertisers be able to browse their blog then buy ads, or earn on PPC from showing text ads

Amusingly, this blog network doesn't appear to have a blog

Expo Active

Description: Lord knows, there's no way to tell without signing up
Warning: Website looks like it was thrown together in about 10mins. Very worrying.

Fast Click

Description: Variety of different formats including regular text ads

Feedburner

Description: Various kinds of banners and text ads for RSS feeds

Feedster

Description: Similar offerings to Feedburner, text ads and banners in feeds

Federated Media

Description: John Battelle's ad network for blogs. Banner ads in various sizes.

Info Grabber

Description: Autogenerated contextual links to affilate sites advertising with Commission Junction / Share A Sale

IntelliTXT

Description: Links words in the body of your text contextually to advertisers

Kanoodle BrightAds

Description: Contextual Adsense like ads

Kontera

Description: Very similar to IntelliTXT

Mirago Context Stream

Description: Mirago is a third tier PPC engine, it can deliver contextual PPC results into content

Nixxie

Description: UK specific IntelliTXT like product

Pheedo

Description: Provide "tools" to integrate ads into regular and feed content

Quigo AdSonar

Description: Private label PPC bidding for text ads on your site

Note: Must have 1M+ page views a month to join

Revenue Pilot

Description: Provides a PPC Search solution for sites. Possibly a little too complex for most busy bloggers to be bothered with.

Search Feed

Description: Search and content PPC text ad integration. Like Revenue Pilot, a little complex

Target Point

Description: Contextual affiliate text ads payed out on a CPC basis

Veoda

Description: Auto generated PPC text links provided by other third tier PPC engines

Yahoo Publisher Network

Description: only open to usa based webmasters.