Check your app store. They're of limited use, but as long as you only use the measurement as a reference for your personal use they're handy. I use Sound Meter for Android. It maxes at 100 dB on my Motorola Droid RAZR and on my previous Droid X. Other brands of phones max out at varying levels, some of which are so low they really are useless for the car.
makeitlouder.com has an SPL meter that's as accurate as the DMM you hook it up to. If you don't own a good DMM it's probably worth getting that already calibrated with the sensor from him. That's your least expensive option that's accurate and good up to real competition level SPL.
There's some RTA software that you can pirate (or pay for) but will require an accurate microphone and a good sound card to make work. Google is your friend there, various DIY audio sites have volumes of reading on the subject of rta software and hardware options.