UX Booth recently did a posting on tools that help with web accessibility. I ran my personal website through all of the tools and found issues. (Special joy.) I particularly found the image analyzer from Juicy Studio to be quite informative. They have some other tools and links available on their website.

An article from UXBooth turned me on to Browsershots. Browsershots collects a group of screenshots of your website using different browsers and lets you view and download them. It's the coolest thing. And useful too... it turned up four errors on my site in a couple of open source browers.

For some common web elements (like dropdowns, flyouts, or ads) there is a timing component to your testing. Often they are available for a short period of time after you move off of them. This allows people who might have shaky hands to still use those controls. But how long is long enough? Or too long? When you think about your testing for these seemingly simple elements, think about the timing and make sure you're calibrated correctly for your target audience.