If you're looking for a person to person video conferencing software, you can download Skype
for free and use the video chat feature. Of course, participating users will need Skype and a Skype account.
If you want multi-user video conferencing capabilities, then you can download and install a free plugin for Skype called Festoon
(used to be called vSkype) which 'provides fast real-time video between you and all of your Skype contacts'.