I just put these together. Note that these are intentionally kept vertical-agnostic.

High-scope Infrastructure - use domain knowledge and innovation to provide a turnkey solution to a broad array of customers. Customers outsource to the service instead of attempting to roll their own equivalent. The company gives them a better solution than they could build for the same price. These startups usually ride on a new innovation and are capital intensive. (Amazon EC2, AppJet, Skype, Heroku, Sendgrid, Dropbox, iPad PoS)

High-scope Web App - as above but the utility is delivered via a traditional web app. These companies are still 'high-scope' even if they only provide services to a particular vertical, because their solution is usually more general than an equivalent in-house one would be. (Basecamp, Hoodle, Freshbooks, Convore, Wufoo, Xero, most SaaS)

Direct Marketplace - facilitates real commerce directly between two parties, where one or both can be a consumer or business. Users are attracted to these services because they can trade goods or services. Reliance on network effect. (eBay, Etsy, AirBnB, limos.com, Smarkets, Groupon, Kickstarter, 99designs, Theme Forest, GumTree)

Ad Marketplace - as above but for ads only (Gumtree, Craig's List)

Ads with peripheral service/content - visitors are attracted by the utility of a service or by engaging content, revenue from on-page ads. Even though the 'peripheral service' takes significant effort, the underlying aim is to drive traffic for the consumption of ads (Facebook, Google, SongKick, about.com, justin.tv, Reddit and other community sites)

Do you agree with these? What would yours be?