PocketStrings
If you want to practice the guitar, but often find yourself in less-than-guitar-friendly places (class, conference rooms, the jury box), PocketStrings ($25 and $30) might be just what you need. It comes in two models, one with four frets that's great for playing chords, and one with six frets that's perfect for scales. It has genuine guitar strings and a strum pad to keep your rhythm right — so it feels almost like the real thing, only without the noise. Collapses down to fit comfortably in a pocket or bag.
