Yahoo! Search Marketing, formally known as Overture. This was the first large PPC network and is now second to only Google's Adwords.

Adbrite is a quickly growing as one of the biggest challengers to Adwords. It has fewer restrictions than Google, such as allowing gambling websites to advertise.

7Search is a PPC search engine with an affiliate network. Their traffic is lower than the others, but there is virtually no restrictions to what you can advertise here.

Google is by far the worlds biggest search engine and they also have the largest PPC network. They can offer you endless hits, but the price isn't always cheap.

VisitorFinder provides a large selection of bulk traffic options. They can deliver you 10,000 visitors from only $17.95.

PopInAds delivers your ads in the form of pop-ups. They can deliver you 10,000 visitors from only $21.00.

Axandra is the creator of the tool Top Ten Optimizer. This software includes keyword tools, a rank checker, search engine subitter and more.

Link Assistant is an SEO tool that will help you cut your link building time. They also build a rank checker and SEO SpyGlass, a tool for analyzing your competitors.

Link Building Direct provides linking services. They will help you with creating backlinks and do link exchanging for your website.

iNeedHits provides services for improving your rankings with the major search engines. They have optimized over 20,000 websites since 1996.