HTML Validator
I do a fair bit of LAMP programming at work and one of the tools I hold absolutely invaluable in this regard is HTML Validator. This handy-dandy little program is a Firefox Extension that is based on HTML Tidy and does w3c html validation. Just right click on the website and pick “view page source” and you are presented with not only your source, but also some great validation info and even a way to automatically generate w3c compatible html right there. This util is really great and I use it all the time. Try it yourself, you might just like it too!