Link and HTML Validation
Most web browsers are very forgiving when it comes to improper page coding and bad URLs. Search engine spiders, on the other hand, are picky little critters that tend to rank your site badly or not at all if it has too much of either. Thus, it becomes very important to regularly review your website for compliance with current standards. This is, of course, an important web design standard to practice anyway, but its effect on search engine placement lends it even greater weight.
Thankfully, the same organization that writes the standards provides useful online tools to help with the process. The World Wide Web Consortium provides a Markup Validation Service, Link Checker, and a CSS Validation Service to validate your HTML, links, and style information respectively. Use all tools to ensure that your site is compliant and spider-friendly.