There exist several apps with test tone generator functions. This FreqGen app is simple to use. Read here. I did find this app useful i.e. to start up a FM repeater with a 1750 Hz tone call. The modern FM rigs has this function in-built, but sometimes it is hard, without the manual, to remember how to do it and of course when using a radio without this function. In this video you can see and hear when I used this app to start the FM repeater SK0RDZ, some 60 km away from my home QTH, on 145.650 MHz with a 1750 Hz tone call. The microphone of an Yaesu FT-857D was held near the iPod Touch gen 3 loudspeaker.
Another app that I have found useful is the DTMF Pad for generating DTMF tone pairs. Normally you use a microphone with DTMF buttons i.e. to access FM repeaters all over the world via the EchoLink system. But if do not have such a microphone this app is very handy. In this video you can watch how I used this app to randomly access a repeater connected to the EchoLink system. I just held the transceiver´s microphone close to the loudspeaker of my iPod Touch gen 3. The repeater I used for this demonstration was SM0HGS/R (node 7485) on 145.725 MHz in Stockholm, Sweden and KD0IZY in New London, Iowa, USA was randomly accessed.