Sunrizer: $5.49 AU
Sunrizer synth is a virtual analog synthesizer that takes the definition of iOS synthesizer to the next level. Thanks to carefully designed architecture and heavy usage of co-processor it blurs the boundaries between iOs and hardware synthesisers. Sunrizer was built to perform and works perfectly with any MIDI keyboard or sequencer.
"These apps also have support for Core USB Midi which allows the connection of a USB Midi keyboard through the USB Camera Connection Kit adapter so that the synth can be played from the keyboard. External speakers can be attached through bluetooth or the headphone out on the iPad.
I have a Yamaha PSR S500 keyboard with USB Midi and have successfully connected to the iPad to use the apps. There are some expensive adapters for connection of keyboards to the iPad, such as iRig, V-Midi etc. but if you want a simple connection, the Camera Connection Kit works fine. Obviously it is going to run the battery down a bit, but for casual use it 'just works'.
Another new app is Fruity Loops, FL Studio HD for music creation via the sequencer method."
FL Studio Mobile HD: $19.99 AU