SEO Bulletin

Use this Keyword Difficulty Check Tool to see how difficut it would be to rank for specific keywords, or keyword phrases.

http://www.seologs.com/keyword-difficulty.html

A look under the hood: link equity explained.

Here is an easy tool that tells you how fast your page loads (among other things): Website Speed Test

Other features include backlink checker (with PR), future PageRank.

If you want to validate a XML file here are a few tools:

XML Syntax Checker

XML well-formedness checker and validator

XML Validation Form

Prevent others from sending spam using your domain name.

Frequently asked questions about SPF:
http://spf.pobox.com/faq.html

Check SPF records:
http://www.dnsreport.com/
http://www.anti-spamtools.org/SenderIDEmailPolicyTool/Default.aspx
http://www.dnsstuff.com/pages/spf.htm