I have been searching the app store for days looking for a tool to assist with song writing whilst on the road - no, not whilst driving but touring or in an aircraft. Believe it or not the best option for this is Apple's Garage Band. Before you laugh at what you may think is a "toy" I challenge you to find a more comprehensive tool, with as many functions, ability to actually play parts, a sampler and audio record for vocals, effects . . . it just goes on and on. Now the most attractive aspect if you've already spent $100k on your studio like I have with ToKwerX; the app costs $5.99. Seriously, I recommend it for the ability to put down an idea but actually have some pretty serious production right at your fingertips. Check it out.
Song Writing Tool