The Mbira
Thumb pianos are used throughout Africa. The most known of these is the Kalimba, which has a single V shape of keys that alternate left and right ascending the scale. But the Zimbabwean variety, the Mbira, is a little more like a piano in that the left hand has the bass notes and the right has the trebles. The above is a photo of my trusty Rinos Simbotti mbira with the keys labeled so you can play the below tunes (but only if you have an Mbira at hand...).
I bought my mbira from my friend Fatima, who studied with Rinos and sold enough mbiras for him that he made a house with the money she sent him! While I know a few traditional tunes and play with traditional technique, I mostly play my own music on it and use precise Blue Tac blobs on certain notes to create different tunings (see below). To amplify it, I either lean it on the bridge of my guitar (or acoustic bass) or rubber band the capsule of a Behringer XM8500 mic onto it, which gives a lovely sound. I find using a piezo pickup doesn't get the bottom end well enough.

U4 M3 U1 L3,   U4 M4 U1 L5,   U3 M4 U2 L7
U4 M3 U1 L3,   U4 M5 U2 L6,   U4 M4 U2 L7
U5 M2 U2 L4,   U5 M5 U2 L6,   U4 M4 U1 L7
U4 M3 U1 L3,   U4 M4 U1 L5,   U3 M2 U1 M1
M1 U1 M2 U1,   M5 U3X L6 U3X,   M2 U2   L4 U2
M1 U1 M2 U1,   M5 U3X L6 U3X,   M4 U3X L5 U3X
M1 U1 M2 U1,   M3 U4   L3 U4,     M4 U3   L5 U3
L7  U2 M4 U2,   M3 U1   L3 U1,     M4 U3   L5 U3
Here are two traditional Mbira tunes as I was taught them by Fatima and Elysia: There seems to be many subtle variations on both tunes. U3X means U3 and X played simultaneously. I've grouped the notes in 4's for ease of learning, but traditionally the rhythm is triplets, with the pulse landing on the underlined notes. Occasionally it's played with the pulse on the note before the underlined ones (and rarely the one after, although it is nice). I should mention that the first tune was pinched and played on piano by Penguin Cafe Orchestra under the name of "Cutting Branches For a Temporary Shelter".
To hold correctly, insert the right pinky finger downward through the hole and wrap the right ring and middle fingers around the side, such that the right thumbnail can pluck downwards on keys X and C1-5, while the right index fingernail can pluck upwards on keys U3-9. All the other keys are played with the left thumbnail, with the left pinky finger hooked under the bottom edge of the wood and the other fingers around the side.
Tuning: Mbira tuning requires patience. To tune down a key, tap brutally on the top end and to make it sharper, knock the key out (upwards), use pliers to bend the top end down a little and then tap it back in gradually until it drops to the desired pitch. I'm always surprised how in tune mbiras often are when you find them in artifact shops: It usually doesn't take much to knock them into a nearby concert pitch. A's, G's and C's are common, but the occasional pesky Eb pops up now and again. There are several traditional Mbira tunings, but there are two main ones. One is simply a major scale (using C as a general tonic note, although my mbira is actually in A) in this ascending pattern: L1=C, L2=E, L3=F, L4=G, L5=A, L6=B, M1=C, L7=D, X=E, M3=F, M2=G, M4=A, M5=B, M6=C, U1=C(same as M6), U2=D, U3=E, U4=F, U5=G,, U6=A, U7=B, U8=C, U9=D
The most common tuning is the same as this but with flattened 7ths (keys L6, M5 and U7). The above tunes work in both tunings, but the second more often has the flat 7ths. There's another tuning that is quite common that is a phrygian mode (major scale starting on the 3). Most of my songs use the straight major tuning. I sometimes use different tunings by flattening one or more notes by adding a blob of Blue Tac under one the key (which sure beats getting out the pliers every time. My song "Carrot" has only the L6 key flattened, which opens up the palette of available chords a little. My song "Dodgy" has L2, X and U3 flattened (all the 3rds) which makes a "jazz melodic minor", although, since the song is more in D (the 4th in the key of A), it's a "lydian dominant" scale (#4, b7).
While an Mbira sounds just fine on it's own, it's traditionally amplified by adding any variety of rattles and/or placing it in a resonating gourd. Other resonating objects that can be used to naturally amplify Mbiras include guitar bodies (leaning on the bridge is best), woks, drums, furniture, slow moving animals and doors (providing a lovely alternative to boring old knocking). To amplify the mbira, I find using a stick-on piezo pickup doesn't get the bottom end well enough, but leaning it on the bridge of a guitar (or acoustic bass) which has a piezo pickup in it that is under pressure give a much better sound (as well as the added acoustics of the guitar body). Alternatively, I rubber band the capsule of a Behringer XM8500 (dynamic) mic onto it, which gives a lovely clean sound.