HTML comments, in addition to being useful (as iandunn points out) and an easter egg (as Mouthpiece observes) can also be downright hilarious. See, while Al Gore purported to support the open source movement through comments in his campaign's web site, some other sites aren't quite as nice. Some use right-click traps in custom browser windows to mask their source code, but those can be defeated. Earlier this year, someone posted on the newsgroup the web site of the National Academy of American Scholars, an organization whose mission statement is

To encourage a national discourse in order to raise the current educational level of pre-level college students by setting high academic standards and offering incentives to surpass these standards.
which would explain the attacks against Carnegie Mellon University, calling members of the academic community Communists as an insult, and various odd warnings on the front page. Don't attempt to duplicate the web site's look and feel. "it is recommended that both browser and Platform be 32-Bit architecture."

Just to make sure that you don't try and duplicate their web site, the following comments appear in their HTML source. (Source of

<!--Viewing this source code is an invasion of privacy.
Please exit viewing this source code immediately. 
This code was created, edited, and authored by Team N.A.A.S.
for National Academy of American Scholars. Downloading this source code
or any source code authored by N.A.A.S. is a violation and direct
infringement of N.A.A.S. copyright privilege. 
<!-- hide code from java-impaired broswers
// Create custom JavaScript 
/*Viewing or downloading this souce code represents
 an invasion of privacy and will be prosecuted as such.
All N.A.A.S. proprietary code is tracked by N.A.A.S. 'spiders'
customized 'robots'. 
Downloading this source code into your host system
is a severe security risk and health hazard to your computer system. 
//You have been forewarned!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
(some JavaScript code for navigation)
//hide executable files*  * * * * *

Immediately after viewing this source, my computer promptly exploded. Damn you, NAAS!