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