Korg AW-2: good option
It works well. I usually use it on the piezo (contact) tuning mode, so it's picking up only my instrument. However, if I'm not playing it may pick up sympathetic vibrations of other instruments and show those tunings as well. I clip it onto the bell of my trombone where I can easily glance at it (during longer notes) and check my tuning. It can be switched to an ambient ("Mic") mode, allowing tuning readings for instruments to which it is not attached -- or for clipping it onto my stand.