From Shape Services, the makers of the popular IM+, comes Persistent.im, basically an always-on private conference that allows you to use your current IM client.
Each conference shows up and behaves as a contact in the IM contacts' list. Conference participants may use accounts in AIM®/iChat, MSN®/Windows Live™ Messenger, Yahoo!®, ICQ®, Jabber®, Google Talk™ and MySpaceIM and keep using their preferred instant messenger client.
You can use multiple IM clients and load your contacts from your favorite IM service, and there are a few bot commands to make conferencing command line-like friendly.
The free service works across desktop and mobile devices - wherever you choose to IM from.