Agreed with all on here.
I did this - hindsight is a great thing and now I can see valid reasons as to why I shouldn't have. Granted it was a few years ago, but it many respects it was a very bad decision and I was young and pretty dumb.
The salient point is that if the operator is asking / offering you "free" hours (i.e. not wanting to pay you a wage) then what else are they doing on the cheap? Just be very sure what you are getting in to. There are some great operators out there (although they would probably pay you) and there are unfortunately some that are rotten to the core.
Due to this awful recession, I do some part time flying when I can - very little pay but my expenses are covered at least. Maybe you could at least ask for that?