Use SIPE and OCS with bitlbee-libpurple

Bitlbee + libpurple can talk to SIPE and Microsoft's OCS. First, you need a Bitlbee with libpurple support. Then it's easy. For OCS, you add your account like this:

account add sipe EMAIL-ADDRESS,DOMAIN\USERNAME PASSWORD

Pidgin calls the EMAIL-ADDRESS field "Username" and the DOMAIN\USERNAME field "Login." For non-OCS SIPE you might not need the ,DOMAIN\USERNAME part.