Validating an entire site
Using the W3C validator to check a single document is easy. But what if you need to validate an entire site consisting of hundreds or thousands of pages? Read Validating an entire site at Standards-schmandards, where Pete explains how to install the W3C validator and use it to batch validate entire sites.
Excellent stuff, and it caught a couple of errors that had managed to sneak their way into a site I'm currently working on.