I found that for me a 50mm wasn't always enough for concerts, as I sometimes got very close to the stage, so in addition I've gotten a 2.8 Tamron 28-75mm. It's wide enough to get decent shots even when you're right up front, and if you're not you can zoom a bit. Of course, the 50mm 1.8 is preferable if the focal length works for where you're standing!
I seem to be taking good photos even if it's only F2.8.
For a walkaround lens, I'd recommend getting a lens that starts at 17-18mm instead of one starts at 28mm. I'm considering getting the Canon 17-85mm IS.