Google Webmaster Guidelines
Google's definitive advice and recommendations for webmasters. Follow their guidelines, and you won't go wrong.

Google Webmaster Central blog
The official blog of the Google Webmaster Central team, and a great source for tips and advice on building Google-friendly sites. This is where Google announces new features and shares the latest information they believe to be useful to webmasters.

Google Webmaster Tools Discussion Forum
If you have a question about Google's Webmaster Tools, chances are somebody else has asked it already and you can find it here.

Google Webmaster Help channel on YouTube
Helpful videos on all aspects of Google's Webmaster Tools. New videos are added frequently, so check back often.

Google's Search Engine Optimization Starter Guide
A detailed guide to best practices in search engine optimization, with plenty of illustrations and examples.

Google for Webmasters
A detailed overview of how Google crawls and indexes the web, and serves search results.

Google's Free Keyword Tool
Use Google's free Keyword Tool to get new keyword ideas.

MSN (Bing) Guidelines
All webmasters need to know about using MSN (Bing), including how the MSNBot works, guidelines for successful site indexing, and usage information for Bing Webmaster tools.

Yahoo Guidelines
An overview of Yahoo's content quality guidelines for successful site indexing.