Description:
This tool takes two or more props and welds them together.

The difference between this tool, the smart weld tool, or the multiparent tool is that this tool literally combines the physics meshes of each prop together.
What does that mean? For all intents and purposes, your props are now one single prop.
Watch the video for more information.

This version does not crash the game like the old version and is much more stable.
Useful for Flood, DarkRP, and any Sandbox-based gamemode. Will cut lag almost entirely from servers which typically have lots of unfrozen welded props.
This version also includes more features as a tool.

Duplicating polywelded objects should work to an extent, but players who paste the dupe must have this addon.
Saved games will not maintain the polyweld yet.

ConVars:
Use sbox_maxpolysize to change the maximum amount of props which can be PolyWelded at once. 100 is default.
Setting this too high might allow minges to crash the server with 400-prop PolyWelds. It's not THAT stable...

Attribution:
Some credit to Sir Haza for making the original tool, even though the code is hardly the same in this version.

Facepunch:
http://facepunch.com/showthread.php?t=1438254

Enjoy!