The Web Standards Project: Check, please!. Checky is a Mozilla add-on which facilitates document validation through one of 18 different services. The validation service is selected through the context menu and the current page is validated in a new window or tab. What’s more, you can bundle several services to run at once through the Checky-Agent. Damn, that’s sweet.
The downside is that some of the services, such as Mark Pilgrim and Sam Ruby’s RSS validator, do not perform auto-discovery and so end up validating the page itself, not the linked documents. As a result, you cannot configure the Checky-Agent to automagically validate your HTML and RSS in one go. Arguably, Checky should be auto-discovering these links and passing that URL, instead of the URL of the current document, on to the validator.