Websites

Pay Per Click Analyst
Quality reviews, up to date news articles, and more.

Vericlix
Free resource to prevent click fraud.

Pay Per Click Search Engines
Over 659 pay per click programs reviewed. Excellent list of pay per click-related forums.

Pay Per Click Guide
More difficult to browse than some, but offers a solid list of marketing and webmaster tools.

Search Engine Guide
A great spot for newbies and broad SEO and SEM resources.

Search Engine Blog
How do pay per click campaigns complement SEO?

Pandia
I love this site. A lot of great links to tools and resources.

Pay Per Click Tips
Exactly what it describes.

Pay Per Click Universe
FAQs, other condensed information about pay per click search engines.

Bruce Clay
An overview, and a chart showing the relationships between search engines.