The Leap Motion controller is very useful for controlling electronic music. Here's the video that got me interested:
 
https://www.youtube.com/watch?v=kLkPvmr93K8
 
I found several contradictory sets of directions on how to set this up, so after I got it working, I decided to write up my own set of directions, specifically for Reaktor Serenade III running under Sonar.
 

1. Buy a LeapMotion controller for about $79.00.
   
2. Register it online.
   
3. Go to the Leap Motion app store, put $10.00 into your account, and buy Geco. (When you download it, it goes into you Leap Motion Apps Home, not your usual download folder.) Play with a free app or two to get used to using the Leap Motion controller.
   
4. Go through the Geco tutorials to learn how to configure it. It's Mac-oriented, so don't forget the tutorial on Windows. (But at the beginning of the Windows tutorial, don't take the business about "%localappdata...” literally. Just find GECO_LM.exe on your drive, create a shortcut for it, and move the shortcut to your desktop.)
   
5. Download and install the freeware Loopmidi.
   
6. Start Loopmidi. You'll need to start Loopmidi and then Geco at the beginning of every session you want to use them in.
   
7. Open Geco and start configuring it. Here's a summary of how I configured it for use with Serenade III:
   
8. File → Document Settings → for Output Port, select loopMIDI Port.
   
9. In the main Geco UI, for Left & Right Position under the third hand, enter MIDI message 22 and Rest Value = LAST. Deactivate the open hand in case you accidentally open your hand as you use it.
   
10. For Up & Down Position, do the same things and use MIDI Message 23.
    
11. For Back & Forth Position, do the same things and use MIDI Message 24.
    
12. Open Sonar and check in Edit → Preferences that loopMIDI Port is checked under MIDI Devices.
    
13. Download Serenade III from the Reaktor user area, and open it in Sonar.
    
14. Back in Geco, select the S under Left & Right. That solos that gesture in preparation for the next step.
    
15. In Serenade, right-click the bow and select MIDI Learn.
    
16. Move your hand left and right over the LeapMotion controller so that Serenade picks it up. Now you're bowing.
    
17. Unsolo Left & Right, and solo Up & Down. In Serenade, right-click Force, select MIDI Learn, and move your hand up and down.
    
18. Unsolo Up & Down, and solo Back Forth & Forth. Right-click Vibrato, select MIDI Learn, and move your hand back and forth.
    
19. Unsolo Back & Forth.
    
20. In Geco, open File → Preferences and for At Startup, check Open last active document.
    
21. Save your Geco configuration with File-Save.
    
22. Rather than figure out how to save the preset in Serenade (how have they made such a basic thing so complicated?), save your track in Sonar as a track template for future use.
    
23. Learn how to air-bow and enjoy playing a high-quality cello emulation. Hint: keep your hand in range and under control.
    

Here are some other reference sites. The ones that don't mention Loopmidi are Mac-oriented.
 
http://10rem.net/blog/2013/08/26/leap-motion-on-windows-81-for-midi-control-and-more
http://www.askaudiomag.com/articles/using-leap-motion-as-a-midi-controller
http://blog.leapmotion.com/playing-a-virtual-violin-with-serenade-geco-midi/
https://jammingsignal.wordpress.com/2014/04/14/using-the-leap-motion-as-a-midi-controller/
 
Mitch I.

Thanks very much for this. How are you finding it in use?

Besides the technical setup, there's the aspect of learning to actually play a new instrument. I'm not nearly as good as Hagai Davidoff, but it's fun and I'm making progress. I don't have a Theremin, but I guess it requires a similar type of skill. Of course with the Theremin, you have the rod to guide you, whereas with the Leap Motion you're just in the space above it.
 
I used the Geco demo tools to get used to it -- putting together robots and plucking flower petals.
 
Mitch I.

 This product would be "so not me". I'd probably spend more time editing my mess ups.  This is a really cool piece of hardware and very affordable.
 
 I have an ipad with some of those programs to use it as an extra monitor, control DAWs.  For me they become gimmicks then wears off and practicality sets in. Now the cry is for DAWs to be more friendly to touch screens.

Mitch.... thanks so much for laying all of this out... Hagai got me excited about the Leap awhile ago with his virtual conducting vid that I saw over at V.I. Control...and I love this new vid...truly amazing how it can emulate reality of bowing/trems. Hope to give this a try during the rainy season when I'll have time at home...Your directions will help emensely...
 
It's funny, just an hour ago while I was reorganizing my studio layout I found myself thinking about the LMC and spitfire... I was thinking "how am I going to assign cc's to spitfire controls, as I've always just rt clicked on a control and touched a knob/button/fader... I was going to google "manually assigning cc functions to spitfire products"...not even knowing if that's what I needed but obviously the LMC is on my mind!
 
And then, I browse over to the hardware, after a bite to eat...which I rarely do (the hardware, that is...), and here you are...detailing your endeavour so far.... WOW! I love those "chills" moments.  
 
Hope you keep posting as it'll be a month or two till I can take the plunge... I've bookmarked this lovely thread.
 
Thanks!