I use a Magnum 3000 u-lock. At 2.16kg (~4.76 lbs) it's no lightweight, but it does make me feel more secure about locking my bike up when I'm away from home.

I also have a heavy chain (4.5kg/9.92 lbs) and mini u-lock (473g/1.04 lbs) combo that I leave on the rack at work. When I have to transport it, it wraps neatly around my seat post.

I don't care about the weight (see my sig), I care about security. Luckily for me I get to park my double-locked bike in a fenced-in, locked compound that is under video surveillance.

That being said, I'd gladly leave my locks at home if work provided some of these bike lockers: