Search Engines

Google's Webmaster Central - comprehensive info about how Google crawls and indexes websites.

Bing Toolbox - submit url and sitemaps.

Keywords

Seobook's Keyword Volume Tool - keyword search volumes.

Google's Keyword Tool - keyword statistics, including search performance and seasonal trends.

Tools

MozTools - Research with Open Site Explorer.

Reporting

URLTrends - monitor a website's PageRank, Alexa Rank, incoming links, listings in DMOZ and its popular search terms.

Link Building

AboutUs - Website Directory Wiki.