the google friend connect also seems to work very well. Especially in our case where all our employees will be given @example.com usernames that will allow them to sign in to the google friend connect service. This user interface is much prettier than steegle's, although his method of the forms+spreadsheet is very good.

(The particular thing I do not like about htmlcommentbox dot com above is where the comments are stored, and how that might be compromised and would result in our site being compromised.)

I found the post here...
                     http://www.google.com/support/forum/p/sites/thread?tid=585c5ec8c9010383&hl=en

Steegle has instructions on how to do this here...
                     http://www.steegle.com/websites/google-sites-howtos/google-friendconnect-visitor-comments-web-elements