I use Run Buddie but it wasn't free. I don't know if you're prepared to pay something. It's also a general jogging app which functions with beeps for the timings you programme into it, so you would need a C25K written plan too. The advantage is that you then have the app for use once you've completed the 5K. But my best recommendation is for a podcast done by an irritatingly upbeat American called Rawwburrrrt ie Robert. He does a podcast for each of the 9 weeks of the programme. Yes the accompanying music is terrible, but he's great at 'you can do it' type motivation.
I don't mind paying a few quid. I only need it to go up to 5k at the moment - I actually am really not keen on running and don't intend to get into it much but a friend has challenged me to do a triathlon with her in June so I need to be able to run 5k pretty comfortably by then.
I don't think I would get on with cheesy American motivation. A simple 'run' '1 minute until walking' 'walk' type instruction is what I am after.