Most people would claim that Doom3 is the latest anti-productivity software, to which I'm partially inclined to agree.  The game definitely conveys "presence".  I'm going to reserve my thoughts about this for now, other than to say that the "out of the box" multiplayer experience leaves much to be desired.

But the true anti-productivity software I'm talking about is called Novodex Rocket.  It's not a game.  Rather, it's a software product that demonstrates all the features you can get from using the Novodex Physics SDK.  I found myself toying endlessly with trebuchets, Jenga block stacks, horses careening through brick walls (no horses were injured during the simulation), and rag-doll like monsters and stick-men.

One of the other cool features is that this is the first publicly-available product I've seen that uses the new "Custom UI" feature found in the DirectX SDK. 

Go check Rocket out, but don't blame me if your productivity takes a dive.  I've given you fair warning :)