In case you are wondering why you have to install a script on your server to enable these functions is because Yahoo and MSN limit the number of API requests made per day on an IP address basis. So this will make the requests from your server IP.
www.google-dance.com [Free] - This tool allows you to search multiple datacenters at the same time. Warning! Use this tool sparingly - it may violate Google's Terms of Service.
MarketLeap's Link Popularity Checker [Free] - Compare the relative IBLs indexed by several search engines for your site against up to three other sites. Warning! Use this tool sparingly - it may violate some search engines' Terms of Service.
McDar SEO Tools [Free] - Offers several ranking, keyword and link popularity checking tools. Warning! Use these tools sparingly - they may violate some search engines' Terms of Service.
SEO / Webmaster Tools
Check Server Headers Tool [Free] - Check HTTP Status Codes for any URI (includes bulk check feature). Useful for checking for 301 and 302 redirects.
Search Engine World's Misc. Tools [Free] - Page size checker, spider emulator, html validator, robots.txt validator and more.
Xenu Link Sleuth [Free Download] - Check your site for broken links. Quickly check other sites for links to your site.
Keyword Density Analyzer [Free] - Compare the keyword density of your site against another site.
Display Emulators and Validation Tools
Lynx Viewer [Free] - See how your web pages look in a text-only browser. This is how most search engine spiders will see your page.
MarketingProf's Wireless/Mobile Device Emulator [Free (requires registration)] - Article includes link to download (for Windows NT, 2000 or XP) a wireless/mobile device emulator. See how your web page is rendered on internet enabled cell phones and PDAs.
W3C CSS Validation Service [Free] - Validate your CSS code and fix errors that might cause problems with browsers in displaying your page correctly.
W3C Markup Validation Service [Free] - Validate your HTML code and fix errors that might cause problems with spiders or browsers in interpreting your page correctly.
Miscellaneous SEO/Webmaster Tools
Search Engine Relationship Chart [Free] - This interactive reference chart shows you the relationships of all the major search engines, PPC providers and directory feeds.
AdWords Keyword Permutation Generator [Free Download] - Generate bounded keyword permutations for pasting into AdWords keyword lists.
File Splitter [Free Download] - Easily split large text files (like monthly server logs) into smaller, more manageable files that can be completely loaded into Microsoft Excel for detailed analysis.
Profit Protector HTML Tool (Affiliate Link Encapsulator) [Free Download] - Generate Javascript encapsulated HTML code to protect your affiliate links from abuse.
E_Cloaker 2.0 [Free Download] - Generate unicode e-mail link HTML code for your web pages.
SEO Consultants' Advanced E-mail Link Generator [Free] - Generate unicode e-mail link HTML code for your web pages (includes CC:, BCC:, subject and body text options).
SEO Discussion Forums
Please note that there are a lot of SEO related discussion forums available on the internet. If you are just getting started, they can be overwhelming and often offer conflicting advice. The following list is not intended to be a comprehensive directory of forums, but rather a list of quality places where anyone can go and learn with a minimum of hassle or confusion.
High Rankings Search Engine Optimization Forum [Free] - Friendly and dynamic forum with top experts and a web marketing focus.
iHelpYou Free Search Engine Forums [Free] - Focused on industry best practices in search engine optimization.
SEO Chat Forums [Free] - Fast paced forum that with a large membership that explores the technical side of the search engine algorithms.
SEO Articles and Miscellaneous Resources
Search Engine College [???] - Offers classes and certification in SEO & SEM from top experts in the field.
GoogleGuy Says [Free] - Collected postings of an anonymous employee (executive?) from Google.
Inside Out Marketing [Free] - Web marketing articles by Dan Thies.