Free SEO Tools

Backlink Investigation

1. Open Site Explorer: http://www.opensiteexplorer.org/
2. Majestic SEO Site Explorer: http://www.majesticseo.com/
3. Ahrefs Site Explorer: http://ahrefs.com/
4. Link Diagnosis: http://www.linkdiagnosis.com/

Server Investigation

1. BuiltWith: http://builtwith.com/
2. Pingdom Full Page Test: http://tools.pingdom.com/fpt/
3. Google PageSpeed: https://developers.google.com/speed/pagespeed/insights
4. URI Valet: http://urivalet.com/
5. SpyOnWeb: http://www.spyonweb.com/

Website Crawling & Link Checking

1. Screaming Frog SEO Spider Tool: http://www.screamingfrog.co.uk/seo-spider/
2. Xenu’s Link Sleuth: http://home.snafu.de/tilman/xenulink.html
3. W3C Link Checker: http://validator.w3.org/checklink

Content & Markup Analysis

1. SEO Browser: http://www.seo-browser.com/
2. Copyscape: http://www.copyscape.com/
3. W3C Markup Validation Service: http://validator.w3.org/

Keyword Research

1. Google AdWords Keyword Tool: https://adwords.google.com/o/KeywordTool
2. Wordtracker: https://freekeywords.wordtracker.com/
3. Ubersuggest: http://ubersuggest.org/
4. Latent Dirichlet Allocation Content Optimizer: http://www.virante.org/seo-tools/lda-content-optimizer

Competitive Keyword Analysis

1. SpyFu: http://www.spyfu.com/
2. SEMRush: http://www.semrush.com/

Social Discovery

1. Shared Count: http://sharedcount.com/
2. SocialMention: http://socialmention.com/
3. Follower Wonk: http://followerwonk.com/

PR & Outreach

1. HARO – Help a Reporter Out: http://www.helpareporter.com/
2. Blogger LinkUp: http://www.bloggerlinkup.com/
3. MyBlogGuest: http://myblogguest.com/
4. DoubleClick Ad Planner: https://www.google.com/adplanner/

Analytics

1. Google Analytics: http://www.google.com/analytics/
2. Open Web Analytics: http://www.openwebanalytics.com/
3. Piwik: http://piwik.org/

Search Engine Guidance

1. Google Webmaster Tools: https://www.google.com/webmasters/
2. Bing Webmaster Tools: http://www.bing.com/toolbox/webmaster/

SEO Analysis

1. WooRank: http://www.woorank.com/
2. Lipperhey: http://www.lipperhey.com/
3. Pearanalytics: http://www.pearanalytics.com/