http://marketleap.com/publinkpop - Check your competition status compared with you.

http://www.alexa.com <-- good to find out how well your site is doing...

http://www.suseroot.com/seopen/ <--- Awesome FireFox SEO Plugin

http://www.webhosting.info is a good resource which also has a reverse dns lookup feature whois.webhosting.info

http://www.hosting-review.com
Excellent hosting site reviews. The site shows you who the best affordable hosting companies are. You don't have to spend a mint to get good hosting with good support and reliability.

www.visualware.com (Pinging function)

www.TheSEOKing.com
SEO Tools, articles, news and a directory of SEO-related sites (plus others).

http://datacentertalk.com/

http://www.google.com/

http://www.dnsreport.com/

http://www.dnsstuff.com/

http://www.whois.sc/

http://www.pandasoftware.com/activescan/

http://arin.net

http://cpanel.net/docs/cp/index.html

http://cpanel.net/docs/whm/index.html

http://w3c.org

http://alertra.com/

http://webhostingtalk.com

http://www.metricsmarket.com/

www.xtremecomputing.co.uk

www.datacentertalk.com

www.verisign.com (whois function)

www.symantec.com

http://www.zoneedit.com/lookup.html

http://www.spamcop.net/w3m?action=checkblock&ip=

http://www.nielssen.com/tracerouters/

Thanks
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