Music theory is a stumbling block for many amateur and would-be professional musicians. Major and minor scales are basic, and pentatonics and harmonic minors come up a lot in certain types of rock music, but when it comes to the modes, everything starts to sound like Greek--literally. (Modes are basically a major scale from one key played in a different key, and many have Greek names like Dorian and Phrygian.) If you're forming your first punk band, you probably don't care. But if you want to play with more sophisticated musicians, you have to know your scales and modes inside and out.
A new iPhone app can help. Released last week, the appropriately named Scales and Modes is a simple app that helps you learn these fundamental skills. Pick a root note, then select from one of 23 modes, including some obscure ones like Super Locrian and seven different harmonic minors. Then, choose your instrument--guitar or piano--and Scales and Modes will play the scale slowly while showing you the proper fingering.
It's not perfect--you can't control the tempo, and it didn't remember my preference for guitar instead of piano, so I had to set it each time. But it won't overwhelm you, and it's not a bad deal at $1.99.
Once you're ready to be overwhelmed, however, there are far more complete music theory apps available. Fretboard, for example, offers a whopping 82 scales for every key, as well as showing fingering for 132 tunings on any instrument from 3 to 14 strings. At only $5.99, that's a lot of musical bang for your buck. I also recommend the highly touted Guitar Toolkit ($9.99), which is like a tuner, metronome, and playable chord and scale book all in one, and iPractice ($2.99), which gives you a fun way to practice your fundamentals on guitar.