One of my favorite things to do is play along with backing tracks. I wanted to share my experience with various sources in case some of you are not aware of these options - and if there are more sources that I haven't listed here, I'd love to hear about them!
TomPlay:
I recently signed up for a year. I am enjoying this app so much! It's a sheet music source as well as a play-along app (not all music has this option). With a year's subscription you are allowed to print up to 48 scores (anything additional has to be paid for).
I have an iPad and so far have not had any issues with their app other than one minor gripe: if you are playing along with audio, there is a pink bar that shows your progress through the music (and the pages automatically turn). The timing calibration for this bar seems to be a tiny bit off (ahead). If I needed this to keep in time it might be an issue.
I've been exploring duets, playing along with an orchestra, and checked out some pop tunes as well. For many pieces/songs, there will be multiple arrangements available for different ability levels. All of the scores I've seen so far include fingering suggestions, but you can easily turn that off. You can also make your own notes (functionality is similar to ForScore). I love that you can adjust volume sliders for different components of the audio - for duets you can adjust the 2 parts separately, for songs with vocals you can turn that off, and when playing with the orchestra you can turn off the piano solo (or in all cases - turn the volume down but not off).
MakeMusic Cloud (formerly SmartMusic):
I do not recommend this for piano for one simple reason: somehow they have not managed to develop an app yet! It's only supported on iPads and you have to run if off Safari. I haven't tried it in awhile but I just recall there were some quirks there that I found annoying. I highly recommend it if you play another instrument (or sing) and can use a PC/laptop. I've been a subscriber for at least 5 years I think. I absolutely love it for clarinet, and it's half the price of TomPlay.
iRealPro:
I bought this last year when I was working on a jazz standard with my teacher. It was fun.....but I haven't touched it since. I'm sure it's a great option if you improvise.
Books with backing tracks:
Mike Springer has several different "Not Just Another ___ Book" series. My favorite is the "Not Just Another Jazz Book" There's also Christmas, Praise, and Scale (pieces focusing on scale passages). They are graded level 1-3. I have only purchased book 3 which I'd say is mid intermediate (I wish there were more!). I have the level 3 jazz, Christmas, and scale books. I found the scale book a little disappointing because it has "orchestral" backing tracks, but it's just midi...yuck. Each book includes a CD (probably with an online download option). There's 2 tracks for each piece - the extra is at a practice tempo. I just upload the main one into an app where I can control the tempo for playback. The books are available on Amazon and very affordable.
Rockschool for pop piano: I'm on a classical track so this is purely for fun. It's a graded exam system, but of course you can buy the books without signing up for that. They are a little spendy considering that each level/book only included 6 or 7 songs. They include downloadable backing tracks.