Some very useful online web validators can be found on the World Wide Web Consortium's (W3C) website.
Don't be alarmed if you test your web page and the MarkUp Validator finds numerous errors. Some of these errors may be a result of older web editing software not creating code to the W3C's specifications, but this legacy code is probably perfectly fine with most web browsers. However, the validator can also discover real code errors (like unclosed tags) that can cause web page display problems.
The Link Checker is quite handy for checking voluminous links pages, which can be quite tedious to check manually for broken links. Any links flagged as broken, I check manually just to make sure that they really are broken and not just unresponsive to the automated link checker.