What Are the Best Apps for Savings/coupons/cash Back?

I see a lot of people post deals and vouchers where you have to have a certain app to take part. Are the apps free and in your opinion what are the best savings apps to download? I used to be part of a cashback website but ended up having to pay a monthly charge that I had no idea I was paying so am a bit wary about putting my bank details in to these sort of sites again! Any advice would be very welcome. Thanks