Yeah, you’ve knew it before, there is also Windows Live Messenger for Windows Mobile 6.1, why, you might ask? Simple, It’s basically WLM 8.5 but on a mobile GUI.
And also there was Nokia Chat, simple enough, it also supports Nokia’s proprietary “Ovi Chat” which was bought by Yahoo! in 2011 (they obviously accepted the deal. Luckily, it wasn’t patched out of there until 2011), Yahoo! Messenger, and Google Talk.
Hence the image above.
UPDATE 08/03/2018: MSN Messenger Mobile uses XMPP.