Here's an interesting iPhone App, Audio Tool by Performance Audio in SLC Utah. I've seen various versions of function generators before but this one really caught my eye, beautiful looking interface. They have various separate versions of this but this one has 6 tools:
Here's the full description:
This amazing collection of professional audio tools is a must have for every audio professional, home theater enthusiast, recording engineer, concert goer, and anyone else who needs a fast and easy way to measure the sound levels around them, keep rhythm or tempo, turn their iOS device into a microphone, generate test tones, calculate recording times, or quickly find the definitions of commonly used audio terms.
Audio Tool currently includes these 6 great tools for your everyday audio needs: Decibel Meter Pro, Microphone Pro, Tempo Pro, Tone Generator Pro, Bit Calc Pro and Audio Atlas!
I'm mainly interested in the 20-20KHz sweep tone generator. I soldered up a quick patch cord with 1/8 stereo jack and checked it with a real oscilloscope and the signal looked great, flat from 20 - 20KHz.
I'll be trying this soon when I do some frequency sweeps on my VTA ST70 with various driver tubes I have on order. I do have a HP function generator which I can correlate results with and that I can go way above 30KHz.
But for simple test equipment at $1.99 this iPhone app is great for not much money! Of course to really use this you need an oscilloscope to see the output.