Well below are things that enable polgramy:
Stuffplug
Add's lods of features to msn messenger. Requires messenger plus!
Mess Patch
One of the 3 main patches.
A-Patch
One of the 3 main patches.
Tabbed Polygramy Msn
Tabs your msn accounts.
Ghost Patch
One of the 3 main patches.
Hex editing:
Enable Polygamy
Offset: 0x0013B12B
Normal: 0xB7
Patched: 0xB6
Skins
Some skins add it.