There are various online services offering automated web page checks. pinkjuice.com/check is just a simple tool generating links to the report pages of some of those services, including emulators, validators, and accessibility checkers.
I find it more convenient to fill out one form instead of more than ten when checking a page. And it's handy to have a list of all the results pages, for example when sending a status report to a client about a website that's being worked on.
Many aspects can not be checked automatically, especially the appropriate use of markup, and many aspects of accessibility.
For local files, running local validators (with catalogs) is way faster, and you can always have the latest version.
Results from Page Valet
(Page Valet form at webthing.com (parser options etc))
Results from RXP (XML)
(parser,
validator form at cogsci.ed.ac.uk)
(watch out: the online service seems to cache documents)
Results from W3C Markup Validation Service
(Markup Validation Service form at w3.org)
Results from the W3C CSS Validator
(CSS Validation Service form at w3.org)
You could use www.delorie.com/web/lynxview.html but I recommend to run a recent version of Lynx itself, locally. The home page is lynx.browser.org; for Windows, you can get it from www.jim.spath.com/lynx_win32/.
Viewed with an HTML 2.0 browser (approximation) (at delorie.com)
Results from Accessibility Valet
(Accessibility Valet Demonstrator form at webthing.com)
Results from Cynthia Says
(Cynthia Says form at cynthiasays.com)
Results from the W3C Link Checker ((X)HTML only)
(Link Checker form at w3.org)