Whether you're developing a new site, or enhancing an existing site, these are some of the best free SEO tools I've found.

Analytics

  1. Google Analytics - If you're exposing your site externally, this is a must have.

Webmaster Tools

  1. Google Webmaster Tools - This is another required tool if site is external.
  2. Bing Webmaster Tools - Microsoft's version of Google webmaster tools. It's optional, but recommended.

Keyword Tools

  1. Google Adwords Keyword Tool - Starting point for keyword researc
  2. Google Trends - great for checking query volumes
  3. Ubersuggest - Keyword research ideas.

Toolkits

  1. SEOBook - Has booth free and paid tools
  2. SEO Site Tools - Chrome plugin
  3. MozBar - SEO toolbar for chrome/firefox
  4. SEO Quake - SEO toolbar for Chrome. Obtains information about any site. Helps with keyword competition determination.

Miscellaneous

  1. Builtwith - Shows behind the scenes info for competitor websites
  2. Xenu - Desktop app for finding broken links
  3. Browseo - View your site how a search engine does
  4. Internet Marketing Ninjas On-Page Optimization Analysis - on-page analysis tool to check internal/external links
  5. XML Sitemap Generator - create your XML sitemap
  6. Rich Snippets Testing Tool - See how Google views your structured data markup.
  7. Duplicate Content Checker
  8. Majestic SEO - Link explorer tool.
  9. Open Site Explorer - Link explorer tool.
  10. Ahrefs - Link analysis tool. Track keyword rankings for your site.
  11. Screaming Frog SEO Spider Tool - Desktop app that spiders websites links, images, css, scrips from an SEO perspective.
  12. Alltop - Good for finding topics for potential outreach opportunities (link building).
  13. Visual.ly - Get proven infographics for your topic.

SERP Tools

  1. SEM Rush - View of the competitive SERP landscape.
  2. WhatsMySerp - Keyword rank checker.

Content Strategy

  1. Content Strategy Generator - Helps you plan content strategy intelligently using keyword research
  2. Infographic Generator - Free tool for creating infographics using templates.
  3. Piktochart - Free infographic generator.