W3CValidator - Command Line HTML Validator
The Command Line W3C Validator is a modified version of the very useful online W3C HTML Validator. The intention was to produce a version of the validator that did not require a web server to use it. Indeed, no additional software is required (besides pre-requisites such as Perl).
The use of the command line rather than CGI/mod_perl access means that HTML files can be checked quickly, and possibly in bulk. Also, because it is command line it can be integrated into other applications.
Note: This product is not endorsed in any way by the W3C. The use of the term 'W3C' is only intended to 'tip our cap' at them, and to show the similarity and lineage of the two applications.