A few good apps I used to use and am now reusing are
Olio - people post food that is very close to / at sell by date and it is all free to collect. I've found it great for breadstuff before that can be frozen.
Too good to go - can be a bit sketchy with what you get as sometimes you never know till you've collected but in our area the local carvery have end of day carverys and late breakfasts for £3/£4 and you get a really decent portion!
Top cashback - I always check this if I ever shop online
Bing rewards - takes some time to build but only takes about 10 minutes in the morning to do mini quizzes or polls and there's not necessarily sensitive data shared either. So many points can be converted to gift cards (I usually get £5 m&s once a month)
Swagbucks - it's a survery site mostly so can mine data but I now only use it to do the polls, play the swago board, and I have downloaded the odd game before - when I first joined it was through Martin's money saving site and if you got so many points then they have you a lot more and I ended up cashing about £60 worth of m&s vouchers in the first 3 months. I estimate, due to not doing many surverys now, I'll come out with about £10 a month but again it's quite little time and I do it between work
I'm just getting into being more frugal with food and I've posted some things on the recycling thread about how I'm trying to be more zero waste and hopefully less spendy but I'm looking forward to seeing what others share here
I've got 2 bags of frozen veg on thier way in my big shop order!