http://www.amazon.com/Master-Lock-Pa...dp/B000WEIIQC/

This is what I have, except mine takes a key.

http://www.amazon.com/Kryptonite-Kry...dp/B000NOO3ZQ/

And this steel cable... right, cut the cable, ignore the lock. This is why I want to get titanium steel chain and coat it myself.

EDIT: It's notable that this kind of security is a pretty serious time-based deterrent. You can cut through titanium or probably boron carbide with an angle grinder (might take a bit longer with the boron carbide--that stuff is RIGID); in general, the malleability of the steel will dictate just how hard this is. If it can deform easily, you can deform it by pinching (bolt cutter) or abrasing (angle grinder, hack saw). If it's more rigid, you're better off sledgehammering it; but most rigid steels will flex under force rather than shattering, unless frozen.

So these resist attack, for a time; an angle grinder is rather unconventional, and whoever has that is going to want to seriously steal your bike. Count on a bolt cutter. A hacksaw takes too long, unless your bike's out of sight (or you're in New York).

But like I said, serious time-based deterrent: they're seriously heavy, and if you want the extra several minutes (hacksaw on a boron carbide anything is going to take ages, I doubt I could cut my Master Lock in half an hour but maybe I'll try one day), extra equipment (angle grinder), etc, you need to accept carrying a heavy load.