From Pocket Lint's 'Ten perfect christmas presents for music geeks'.

Musical Ruler

Yes, you read right. The Musical Ruler. Naturally, there's a certain amount of novelty factor to this one but at £6.95 it would make an excellent stocking filler.
The idea is that you put this 30cm device flat down over the edge of a
table and ping the overhanging end to make a note. The ruler has
markings on it for the eight whole tones in a complete octave meaning
that you can basically play any tune.

It comes with a step by
step guide from ruler playing master Dan Wieden and includes tips on
solos, Rock 'n Roll, Folk, Jazz and special effects. Possibly the
silliest present you'll see all year but, when you grow tired of the
fun, it'll still work as a gadget for measuring short distances and
making pens and pencils run straight.

Take Pocket Lint's advice and fill the stocking of the music buff in your life with Dan Wieden's Musical Ruler, available to purchase here.