I would love to have this, and I'm sure a ton of other people would want this as well, but sadly the feature doesn't work. I am, of course, talking about the broken lighting preview. I think it would be an amazing addition to hammer, as you wouldn't need a full recompile just to see if a light looks decent. The Alien Swarm SDK has this feature but it can't be used for full mapping as no TF textures or models work and the lighting works a bit differently. Anyone up to this challenge?