Here are six official search Engine Optimization related resources you will always want to make use of…

1. Google Webmaster Central

This is the one-stop place where you can read the official Google guidelines and get tips for improving and enhancing the traffic to your sites. You should also create a Google account if you haven't already done so. This will allow you to sign in to the Google Webmaster Tools to obtain reports about your sites and how Google sees your web pages.

2. Yahoo Webmaster Resources

This is a useful list of Yahoo resources and guidelines for site owners and webmasters. The provided information include Yahoo search content guidelines and FAQs.

3. Microsoft adCenter Lab Tools

A collection of useful tools for search volume seasonality forecast, demographics predication, keyword research etc. Some interesting tools include a cross-language keyword selection tool (currently seems to support only French) and an Ad-Text Writer.

4. Google Trends

Google Trends displays the search trends for various keywords from 2004 to now. This is a powerful brainstorming and idea generation tool. Spend some time with it every week and check out the trends for your business keywords.

5. Yahoo Site Explorer

Yahoo Site Explorer is the best public tool out there for checking the incoming links to your websites (and to your competing websites). Google provides this information within the webmaster tools after you sign in and verify your sites, but this information is available only for your own sites.

6. The Official Google Blog

This is the place to learn about the happenings at the most popular search engine. Keep an eye on this or subscribe to the RSS feed.