Here is the list for top 10 seo tools and plugins that are free
SEO Quake
https://addons.mozilla.org/en-US/firefox/addon/3036
Open Site Explorer
http://www.opensiteexplorer.org
Redirect Checker
http://www.internetofficer.com/seo-tool/redirect-check/
SEO Book Tool Bar
http://tools.seobook.com/seo-toolbar/
FireBug
https://addons.mozilla.org/en-US/firefox/addon/1843/
Spider Test
http://tools.seobook.com/general/spider-test/
XML Sitemap Generator
http://www.xml-sitemaps.com/
Whois
http://whois.domaintools.com/
Google Webmaster Tools
http://www.google.com/webmasters/tools
SEOmoz GeoTarget
http://www.seomoz.org/geotarget
Source: http://www.stateofsearch.com/top-10-free-seo-tools/