Here's a small one that I built. I made the "bin" out of styrene and filled it with water to check for leaks.
, on Flickr
Here's a lesson I learned the hard way with this one:
Terrariums work when the water evaporates, condenses on the glass and then drips back into the soil. When you have condensation on one side of the glass in the morning, the water level is about right. If the glass top rests outside the "bin" that the soil is in, the water will exit the terrarium and everything will dry out really fast.