This app was mentioned in 3 comments, with an average of 1.00 upvote
Piano Companion Pro to build chord progression ideas. Has a ton of scales. Exports MIDI.
My Piano, it's a virtual MIDI keyboard, multitouch, does velocity, aftertouch, MIDI over WiFi. For shits and giggles record a friend saying something then play a melody with it. Records to an audio file as well.
I don't use Caustic as much anymore. It's so far removed from what I do on a computer I'd rather just get the composition basics down.
I have Cthulu. It's great for noodling around/inspiration (and it is an amazing arp). But nothing beats learning music theory. As for Hookpad, sure you can plop all sorts of chords on there, but do you understand why you want to follow one chord with another? If you do, then go for it, otherwise consider educational resources.
Are you on Android? I like Piano Companion Pro and it's $4. You get reverse chord lookup, too.
Knowing your scales is super important. To help you get there though, I recommend using an app/website like Piano Companion. I recommend paying 5 bucks for the full version. Has a reverse scale lookup given notes. Great to experiment with chords and chord progressions when you don't know their structure off the top of your head.
(Android) https://play.google.com/store/apps/details?id=com.binitex.pianocompanion&hl=en
(iOS) https://itunes.apple.com/us/app/piano-companion-chord-relative/id598574333?mt=8