Moog Animoog - This app from Moog is an amazing value at only $0.99 cents (for a limited time, price is normally $30) compared to its real life counterpart, the Little Phatty ($1400). Add an inexpensive midi keyboard and you have a full on synth with rich Moog quality sounds. Made specially for the iPad, the touch screen adds a unique and more dynamic experience. Unlike Korg and Yamaha, Moog took a different approach with their app by utilizing the unique features of the touchscreen rather than simply replicating the instrument's interface.
Korg iElectribe - I own the Korg Electribe ESX-1 that the iElectribe was modeled after, so I was definitely anxious to check the app out. At $20, it's a steal next to the actual hardware version ($500). iElectribe has a simple 16-step sequencer and with the update to 1.1 you can now record live sessions and export them to your computer. The interface is almost identical and the iPad app is a great way to test the ESX-1 if you've ever thought about purchasing the real thing.
Korg iMS-20 - The Korg iMS-20 app for the iPad does a beautiful job of recreating the vintage analog synthesizer it's based on (the Korg MS-20), right down to the yellow patch cords. Midi support was recently added to the app, so you can now connect a midi keyboard to it. The app also features a built-in 16-step analog sequencer. Once you're done creating a song, you can upload your tunes directly to SoundCloud. You can grab the iMS-20 app for $32.99.
Yamaha TNR-i - The Tenori-on is already an eclectic electronic instrument with a touch-based method of playing. Like Korg, Yamaha pretty much duplicated the actual Tenori's interface onto the iPad, calling their iPad equivalent the TNR-i. You don't get the tactile feel of pressing an actual button, but the app plays and sounds the same. With the Tenori-on costing as much as $600, a $20 app is a small price to pay to experience such a unique instrument.