Pidgin
From Openmoko
Pidgin can be installed from the Angstrom Repositories (see Repositories).
opkg install pidgin
There are various plugins for pidgin provided by libpurple that can be installed to connect to MSN (libpurple-protocol-msn), Yahoo (libpurple-protocol-yahoo), etc.
Do a
opkg list | grep libpurple-protocol
to see a full list.
You can have instant messaging on the go through GPRS.