Q: I'm getting an error when trying to verify my site with a file, even though my file is visible in my browser.
A: Look at the specific error that Webmaster Tools is showing and read the help content for that error. Check that the file returns only the required content (and does not include HTML code, images, or scripts such as counters). The easiest way to do this is to use "view-source" in your browser.
Q: Does it matter whether my Webmaster Tools verification meta tag is in HTML or XHTML?
A: We'll be able to find your meta tag no matter which format you use. Webmaster Tools provides the meta tag in XHTML by default, but you're welcome to change it to HTML; this won't prevent you from verifying ownership of your site.
Q: How do I verify on [whatever platform I use to host my website]?
A: This Knol has instructions for how to verify on a variety of platforms, including Blogger, Google Sites, Microsoft Office Live, and others.
If you host your site on a platform that isn't currently listed, feel free to contribute instructions for verifying on your particular platform!
Q: I'm using a hosting service for my site that prevents me from adding meta tags or uploading HTML files. How can I verify my site?