This is one of the best free ones that I know of.

Spooky Keys 
Here's a youtube demo but it's not the best Spooky Keys Demo.
I love this theremin and just had a look at raptures and there's no way I would swap it with spooky keys.
I've not yet used it with a keyboard though. I recorded it in real time using my mouse which was tricky to say the least but interesting 

There's also fxpansions mysteron which is ok.  



Steve

I use the free VST BTS Theremin. It's got a good assortment of theremin sounds and works great with Sonar.

From what you say the client might be looking for a resonat filter modulated by an lfo sound.  Such were all over the 50s\60s scifi movies.  Simply get a resonant filter going, tracking the key pitch (it doesn't have to be 100%), modulate it w/ a fast (audible) oscillator and amp envelope the sound.  You recognise the sound as soon as you patch it - swooping pure pitch followed by bastard offsprings.  If you've seen a space ship in space from the 50s you've heard it.  Echo to taste.  Easy w/ a VA, and I just used a Z3TA patch that did that exact trick.  Don't recall the name, but it is sooo easy to set up you would be wasting time looking for the patch.

@