Must have app for iPhone / iTouch users...
the Studio Six Digital SPL meter available for the princely sum of $.99! It is a pretty full functioned unit offering both A and C weighting, slow / fast response, and the ability to calibrate it using your own SPL meter. It emulates a classic analog design (down the the simulated screws at each corner) with a quick responding meter. There have been many times when I would have loved to have had the ability to measure level, but alas I don't normally carry around the trusty old Radio Shack meter with me. :)
http://www.studiosixdigital.com/_Media/splmeter1.jpg
They also make an RTA, but alas it is a bit pricier. Then again, there's a free "lite" version I plan to check out.
rw