I'm looking for some kind of forum online where people can freely post
their experiences working for specific corporations, letting others
know whether this is a good or bad place to work.  I imagine such a
website would be full of mostly negative comments, but it would still
be interesting to read.

Can you please tell me if such a website exists?  The ideal site would have a
large number of companies and be relatively up to date and well
organized; don't recommend a website that's hardly ever updated.  No
sites run by recruiters please; must seem like an honest service to the
public

Thanks!

PS if the answer is no--no such website exists, that is an acceptable
answer,  just point me to the closest thing.

A great website is www.vault.com.  This site lists profiles of
hundreds of companies as well as maintains message boards for users to
post comments.  Most of the information is accessible at no cost as
well although there are some services in which you must be a member to
use.  I would say this site fits all of the criteria you mentioned in
your question.

Another site is www.wetfeet.com but this is mostly about consulting companies.

Good Luck!