The AudioKit Synth One is a professional-level open-source synthesizer for your iPad. It has features that match or even rival the ones on iOS synths from Korg, Electro-Harmonix and Moog, which can run upwards of $25. This one, however, was created by a team of more than 100 volunteers around the world, so you won't have to pay a cent for it now or ever.
Once you download the app, you'll get a hybrid analog/FM poly synth with more than 300 presets created by professional sound designers, including the guy behind M83, Tycho and Rihanna, Chase Dobson. It works with Audiobus 3 and Inter-app audio so you can bring the sounds into your other music apps, has five oscillators and 2 LFOs and a 16-step sequencer. You can control the Synth One with a MIDI keyboard as well, and it's got touchable ADSR envelopes to filter your sounds in real time. There's an FM Oscillator with a mod wheel, a mono glide and legato feature and a dedicated sine/square oscillator, as well. You'll be able to customize any sound with a ton of high-pass and band pass filters, Costello reverb and multi-tap delay effects.
As it's open source, the AudioKit Synth One will release its full Swift-powered source code, so other developers can mess around and make their own synth apps. The team plans to add Ableton Link, Roli Seaboard and AUv3 plug-in support.