TM stands for TranslationManager. TM/2 originates from IBM TranslationManager/2 tool and is now available to the open source community. TM/2 is a computer-assisted translation system for translators. See http://www.opentm2.org for more information.
As a long-term user of IBM's TM/2 internal CAT tool, I'm glad they finally realised that the logical step is to open-source it. Under it's "rustic" look and feel, TM/2 is a very capable CAT tool.