What is a validator, and which are recommended?
Validation means a very specific kind of checking - it means checking a document syntactically, and only syntactically, against a formalized syntax specification (DTD), completely. Nothing more, nothing less.
The people at W3C have made a program called Tidy available. You can set up a front-end interface or use 1stPage2000 or HTML-Kit to validate offline. They have TIDY built in.
Tidy does not do a proper validation check. Instead, it checks some syntax issues and partly fixes errors; moreover, it uses fixed checks whereas a genuine validator is happy to validate against any syntax specification you ask, as long as that specification is given in SGML.
CSE HTML Validator, despite the name, is not a validator. It is a decent linter, but it isn't checking your document against a formal DTD, which is exactly what a real validator does. It can be integrated into editor tools like Homesite, Textpad and Dreamweaver, as well as acting as a separate application.
Another good validation check that can be done is the Web Accessibility validation. Bobby is the most recognised of these, although both Accessibility Valet and A-Prompt are highly regarded and recommended
The following editors have a syntax checker built in (not strictly a validation, but useful nevertheless):
- Homesite
- IBM Websphere Homepage Builder
- Dreamweaver 4
- HoTMeTaL pro
DU notes: I highly recommend adding Checky which is a plugin for Mozilla-based browsers allowing the user to check a document with over 20 different known, reliable, customisable validators.