Html Code Validation Tools
Atom & RSS Feed Validator | Web browser tool -Validates atom, rss and kml.
Html Validator | Web browser tool - Checks for common validation errors (free version)
W3C Markup Validation Service | Web browser tool - Validates and checks the markup of web documents in html, xhtml, smil, mathml and more.

Additional Google SEO Tools
Go Fresh | Web browser tool - Due to Googles limited date-based syntax, Goofresh is a way to search for sites added today, yesterday, within the last seven days, or last 30 days.
The tool also shows sites that have added or mention your website links within the same time frame.
Google Fight | Web browser […]

Broken Link Detectors
Broken Link Detector | Desktop software - Link Sleuth checks web sites for broken links with verification being done on static links, images, frames, plug-ins, backgrounds, local image maps, style sheets, scripts and java applets.

Googles Search Tools
Google Alerts | Email - Sends alerts on enay subject via email.
Google Keyword Tool | Web browser tool - Google keyword tool based on Google search data. Includes keyword search and competitor search.
Google Toolbar | Web browser tool - Google toolbar for internet explorer.
Google Trends | Web broswer tool - Provides data on keyword trends including keyword counts, […]

Niche Directories - List Resources
Vilesilencer | Online lists - Assorted directory lists including seo and niche.