It's a promotional button from Messenger Plus!.
It depends on your local and region which promotional buttons are shown and of course if you have enabled them in the Messenger Plus! preferences.
Except for the new logo of Messenger Plus! 5 for Windows Live Messenger, many of those promotional buttons are indeed still using the old colorfull Plus! logo or have a reference to the old colorfull Plus! logo. They can/could not only be seen in Messenger Plus! 5 for WLM itself, but also on the various (promotional) websites from Yuna, like there are http://www.plusgames.com/
However, since the release of Messenger Plus! 5 for WLM, they are slowly updating all those websites too and with that also those old colorfull logos.
For example, the new logo for the Plus!Translator Web application
for the Conduit toolbar is: