I use Real Time tracker 2, its completely free and it works! It does lots of other things as well but I don't bother with them, so many apps seem to have duplicates of what I have already.
Ok, I accept when I power it up it does out of my PAYG data but I've never noticed that it uses much?
I also use very little of my PAYG phone money, (use it just as an emergecy) I make most of my phone calls through the internet and in wifi in pubs etc and when I'm at home.
I'm definately doing one thing this year, I'm switching my voicemail off for good, if I'm really needed they can text me and I'll phone them back!