Google Play
All of my apps on Google Play are now free with a free 3 day trial. Once the 3 day trial is over, you can then decide whether to purchase anything. All app features are free to use during those 3 days, so use the app as much as you like before deciding to purchase.
Amazon
Currently, dFolio, FlickFolio and SkyFolio include the same 3 day free trial as Google Play. My other apps are currently purchase up front with no trial, but that may change in the near future.
iPhone, iPad, and Apple TV
PixFolio on iOS and tvOS is free for iCloud access and basic slideshows. If you want to connect to other services or use more advanced settings, then you need a subscription or lifetime purchase. The subscription includes a 7 day trial so you can try out all app features for 7 days before deciding to purchase.