W3C Link Checker - Link checker PERL source code, via the WWW Consortium (the folks who set web standards); configurable. Handles one URL at a time.
HTML TIDY - Free utility available from SourceForget.net; originally by Dave Raggett. For automatic fixing of HTML errors, formatting disorganized editing, and finding problem HTML areas. Available as source code or binaries.
Big Brother - Freeware command-line link checker for Unix, Windows, by Francois Pottier. Available as source code; binary avaialable for Linux.
LinkLint - Open source Perl program checks local/remote HTML links. Includes cross referenced and hyperlinked output reports, ability to check password-protected areas, support for all standard server-side image maps, reports of orphan files and files with mismatching case, reports URLs changed since last checked, support of proxy servers for remote URL checking. Distributed under Gnu General Public License. Has not been updated in recent years.
MOMspider - Multi-Owner Maintenance Spider; link checker. PERL script for a web spider for web site maintenance; for UNIX and PERL. Utilizes the HTTP 'HEAD' request instead of the 'GET' request so that it does not require retreival of the entire html page. This site contains an interesting discussion on the use of META tags. Not updated in recent years.
HTMLchek for awk or perl - Old but still useful HTML 2.0 or 3.0 validator programs for AWK or PERL by H. Churchyard; site has much documentation and related info. Not updated in recent years.