My bottom bracket may need to be overhauled or replaced soon, but for the moment it's ok. I noticed creaking on every pedal stroke, usually as my right foot came over the top, and only if I applied torque. If I pedalled gently, no noise.
I walked into the LBS, and noticed the guy at the service desk wasn't the head mechanic...bummer. I started to explain the problem to the guy, who seemed a little perplexed, and as I did, the head mechanic happened to walk in the front door 30 feet away, heard the tail end of my explanation from across the room, and yelled to his assistant "tighten the left crank!"
Problem solved, at least for now. If it comes loose again, it may need more work.