Image-Line must have been getting sick of seeing all of the many Ableton control surface apps, so instead of waiting for someone else to make one for FL Studio they did it themselves! ILRemote looks quite deep, and it is free on both Android and iOS!
ILRemote iTunes Description:
Image-Line Remote (IL Remote) is a free Tablet or Phone, user-configurable virtual MIDI controller application for FL Studio and plugins. Just open FL Studio on your computer and IL Remote on your iDevice and the connection is made automatically over a Wi-Fi network.
Control FL Studio instantly or link your favorite instrument and effect plugins just as you can with any MIDI controller. Use a phone, tablet or any combination with up to 15 devices simultaneously.
Use the included controller tabs covering functions including; Transport Controls, MIDI Keyboard, FPC control, Harmonizer Keyboard, Performance Mode (Clip Launcher), Gross Beat FX, Mixer and more. If the control you want isn’t available then you can create your own.
IL Remote allows you to add custom tabs and add controls including Pads, Faders, Knobs, Jog Wheels, Mixer, Clip Launcher, X/Y Controls, Piano Keyboard, Harmonic Grid and Containers. There is a complete range of customization options for each control so that you can create virtually any virtual MIDI controller to suit your needs.
For more details check the online manual here:
Here is a great little demo video. There is also a short tutorial on configuring your WIFI connection.