One of my clients recently asked me how I test my websites to check to see if they have any errors and if they are valid or not. Of course my response was "My sites are perfect, there's no need to bother checking."
I pretty much left it at that with them, but I thought I would share a few of my secret resources with you guys. I'm pretty big on cross-platform standards and these are some of the tools I use to ensure this.
First off, for standard HTML and CSS validation, I use the excelent W3C Validators. If you haven't checked your sites with these, then maybe you should give it a try. If a potential client calls us up and already has an existing website, I typically run a quick check with these tools to show them all the problems with their site. Nine out of ten times, their site has about 20-50 errors.
HTML & CSS
Metatag and Search Engine Related Stuff
And for you hard core programmers out there, you might like this little gem. It's a regular expression tester. Great when your trying to figure those darned things out.
That's about all I use. If you guys have any resources you would like to share, please post them in the comments.