I love boundary mics. Way back in the 90s Radio Shack sold a version of the Crown PZM mic, but with a 1/4" connector instead of the XLR. There's a mod to retrofit them with the original XLR, made easy by the fact that the balanced wiring was used on the RS version, just not connected to the plug. Then they discontinued them and a friend of mine bought out the local store.
They're a great way to pick up an upright piano that's against a wall. Just pull the piano out two feet and hang a pair of PZMs on the wall about 2.5' off the floor and 3' apart. I've heard of them being taped to the underside of a grand piano lid, which is similar.
Some people will mic a kick drum by laying the mic on the damping. In a low room you can use them as overheads that eliminate the reflection off the ceiling. You can tape a pair to a kick drum, one higher on the snare side, one lower on the toms side, for a simple, low profile live mic setup.