Portal -- usually a search engine that may include varied content or services. A high volume of user traffic makes advertising profitable and permits further diversification of site services. A personalized portal allows customization of the interface and content to the user. A niche portal cultivates a well-defined user demographic. [Yahoo!]
Classifieds -- list items for sale or wanted for purchase. Listing fees are common, but there also may be a membership fee. [Monster.com, Craigslist]
User Registration -- content-based sites that are free to access but require users to register and provide demographic data. Registration allows inter-session tracking of user surfing habits and thereby generates data of potential value in targeted advertising campaigns. [NYTimes]
Query-based Paid Placement -- sells favorable link positioning (i.e., sponsored links) or advertising keyed to particular search terms in a user query, such as Overture's trademark "pay-for-performance" model. [Google, Overture]
Contextual Advertising / Behavioral Marketing -- freeware developers who bundle adware with their product. For example, a browser extension that automates authentication and form fill-ins, also delivers advertising links or pop-ups as the user surfs the web. Contextual advertisers can sell targeted advertising based on an individual user's surfing activity.
Content-Targeted Advertising -- pioneered by Google, it extends the precision of search advertising to the rest of the web. Google identifies the meaning of a web page and then automatically delivers relevant ads when a user visits that page. [Google]
Intromercials -- animated full-screen ads placed at the entry of a site before a user reaches the intended content. [CBS MarketWatch]
Ultramercials -- interactive online ads that require the user to respond intermittently in order to wade through the message before reaching the intended content. [Salon in cooperation with Mercedes-Benz]