[whichdan]

I telecommute for a company that uses Skype for everything - we have a company-wide group chat, and separate group chats for developers, marketing, etc. This makes it easy to throw out a quick question or see who's online at any given moment.

Are you strictly looking for web-based software?

[kls]

I would second Skype, they have clients for most platforms and the group video conferencing makes quick developer meetings nice.

[metra]

Web based would be ideal.