The plucked psaltery is a simple musical instrument with a basic but pleasing sound. It's basically just a piece of wood with strings running across it, fretted at one end, with tuning pins to tune the strings.
While the origins of the psaltery are unknown, it must go back to at least the medievel period, and possibly as far back as Biblical times. Medieval engravings show a 'hog-nosed' version of the psaltery.
It also goes by other names: lap harp, Music Maker, Melody Maker, Melody harp, etc., but "plucked psaltery" seems to me to be the most basic categorical term for it. It belongs to the more generic category of "zither".
The most common version of it is the Melody Maker, a child's musical instrument often made in Russia or other parts of Eastern Europe. It comes in a trapezoidal shape, like a triangle with one tip cut short, and has 15 strings tuned in the key of G for two octaves, and thus is diatonic, not chromatic.
It's fun and easy to play: just pluck the strings with your fingers or with a guitar pick to make a louder sound. You can play familiar melodies, or with trial and error come up with more interesting sounds. Like most things musical, more practice and more understanding of music helps to make better music.
It's not very loud, so if you want to perform or record with it, you'll need to look into pickups or microphones to do so. One simple reason that it sounds so good is that after plucking a string, you just let it ring out until the vibration stops, or until you want to play the same note again. Thus, you get a warm, reverberating, echo-y type sound as you strike new notes while the older notes are still ringing. You can get a nice, swelling sound by playing several notes rapidly in succession.
It's even possible to dampen unwanted strings and play full chords, autoharp-style, although this is a bit difficult to do manually. Or use both hands to play more than one note at the same time.
The children's versions are pretty cheap, for about $40 or less. In fact, I saw that Wal-Mart now carries a version for under $20. However, some folk music instrument makers (like Craggy Mountain Music) make and sell higher quality instruments for more money. You can also occasionally find a decent one on E-Bay.
I'd recommend getting a cheap one to try out, and if you really like it, then go for a more expensive, quality instrument. It's a good instrument for musical beginners (children and adults) and fun even for more advanced musicians.