Search Engine Optimisation (SEO) Tools

Search Engines

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

Yahoo's Site Explorer - allows you to explore all the web pages indexed by Yahoo! Search. View the most popular pages from any site, dive into a comprehensive site map, and find pages that link to that site or any page.

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.

Keyword Density - useful for helping webmasters and SEOs achieve their optimum keyword density for a set of key terms.

Yahoo Search Volume - keyword selector tool.

Reporting

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

MarketLeap - link Popularity Check.

Link Building

DMOZ - Open Directory Project.

AboutUs - Website Directory Wiki.

Jayde - B2B Website.

TurnPike - TurnPike Emporium Directory.