Chatango is an instant message application that allows people to post links on Craigslist, Ebay etc. and have people chat anonymously with sellers with no signup. If you have trouble using IM with people behind corporate firewalls, Chatango will work. Signup, grab your username before anyone else does and send someone buried behind a firewall a link in an email.
Its a new product, rough around the edges, but looks like there is quick, free alternative to the enterprise dodads like Liveperson.
Disclaimer: I helped Alec, the guy who did this, with some product design.