Open Source chat software

The idea comes to my mind when i was thinking of project ce-im

Guys Most commonly we use google talk or yahoo messenger for chating

but if i have yahoo messenger and i want to chat with my friend who has gmail id

and i want to chat with him then i will send him a request

but instead of request for chating ,it will send him a invitation for joining yahoo messenger.

so I thought of why not develop a software that can allow you to chat with anyone ,no

invitation and all
What you say?