Like the OP, I had most of the tools I needed for the theater build before hand, but one thing I "borrowed" is a self leveling laser level (it's actually my wife's). That thing has made my life so much easier! I've used it for laying out furring strips for my hat channel, then I used it for laying out clips for my hat channel, now I'm using it for building my soffits, and I'm already planning to use it for my riser and stage. It really makes the job a lot easier when you are working alone, and even with help I would imagine it's extremely handy! The self leveling part is what makes it so handy. I can put a mark on a wall, the ceiling, or the floor, put the laser on it, and I know it's level and plumb. It will put a line on three walls in my room.
Next in line with the laser level would be the framing nailer! Much faster than driving nails!
However, I will go against the popular vote on the impact driver, though. I have a Ridgid impact driver, that I use very rarely. I've found that it's great for driving lag bolts, and that sort of thing, but driving anything up to a 4" drywall screw, it's much faster for me to use a standard cordless. Maybe it's just my impact, but it switches from regular drive to impact pretty quickly, and that makes it slower for driving a screw. Not really a big deal unless you have lots of screws to drive, or you're working over head. Now I'm a pretty big guy, so maybe part of the difference is the amount of force I put behind the drill keeps me from stripping out the screw head, I don't know. But if I had it to do over again, I might have skipped the impact for something else.