Blamsoft makes a killer suite of audio unit effects (AUX) for iOS and a really awesome Moog Voyager emulator. They are all universal now. I recommend getting all of them!
Viking Synth – Blamsoft, Inc. ON SALE
Audio Unit is a type of plug it app that works with host apps.
Host apps are:
Ferrite Recording Studio (Wooji Juice Ltd) *effects only Ferrite Recording Studio – Wooji Juice Ltd
GarageBand (Apple) *instruments only GarageBand – Apple
midiSTEPS (Arthur Kerns) *instruments only midiSTEPs – midi step sequencer toy – Arthur Kerns
MultitrackStrackStudio (Giel Bremmers) MultiTrack DAW – Harmonicdog
VoiceJam Studio (vocal processing, but VoiceJam only acts as a host for VoiceRack) VoiceJam Studio: Live Looper & Vocal Effects Processor – TC-Helicon
Cubasis (iOS DAW, now with AU support) Cubasis – Music Production System – Steinberg Media Technologies GmbH
AUM (audio mixer from Kymatica) AUM – Audio Mixer – Kymatica (Jonatan Liljedahl)
Expand your sound palette by adding Blamsoft Audio Unit effects and instruments to one of those hosts.