Moog came out with an iOS app that's oodles of fun. It's a recreation of the 2 VCO Model 15 modular synthesizer.
Model 15 by Moog Music Inc.
[Only registered and activated users can see links. Click here to register]
What makes it so fun to play with is the authenticity of the low pass filter. It has that unmistakeable mooginess. And it runs on your iPhone!!
When it comes to vintage emulations, other things of note are Arturia's iProphet, iMini, and Korg's iMS-20 mini. The latter also looms large and has a beautiful emulation of the Korg low pass filter. Arturia didn't do as well with the Minimoog emulator when it comes to the low pass filter. The Moog app above is just so stunning.