Recs.
Updated
Logic Pro X is a DAW/MIDI sequencer for Mac OS. It has been in the works for a very long time, and is known for being highly refined and pretty easy to pick up for beginners.
SpecsUpdate
Pros
Pro Free corresponding iPad app
In order to allow more control, the Logic Pro X DAW has added a free iPad app called Logic Remote.
Logic Remote is available in several different views, and is capable of MIDI input and parameter adjustment. It comes with a full library and browser. Navigation is easy as well via the navigation bar along the top of the screen.
Many interesting views are available, such as chord strips. This view allows users to play a series of preset chords that are already in the right key. Gestures are supported with this app, so loop speed and other properties can quickly be adjusted.
This app connects to Logic through WiFi, so the app and program should be running on the same network.