I've had several clip on tuners, and I can heartily recommend the Korg AW1.
I believe Korg has updated the design to a slightly larger AW2, which is in fact slightly improved, but I don't own it. A quick search online makes it appear that they are both readily available.
Plus:
-It's smaller and easier to position than others
-extremely accurate (most importantly!)
-comes with two different clips to allow for different types of instruments (will go from your guitar headstock, to your trumpet bell, to your music stand)
-comes with a great case to hold onto the clip you're NOT using, and for storage
minus:
-the display isn't as bright as I'd like, in some instances it takes a bit of maneuvering to read easily, but the ball joint makes that doable (this is one of the improvements in the AW2, i believe)
-not inexpensive
Every other tuner I've had has failed to soon, or been a bit too big to leave in place, or had a readout that could serve as a flashlight. I've noted that I often see folks playing with this tuner on the headstock, it's so small and light, that it's not an issue to leave it there.
for example
Happy hunting