We needed space for a lot of indoor plants, and we needed to protect them from being nibbled on by our dog or cats. rather than build something from scratch that might not look so good, i bought two detolf glass shelves and added some fans on top.
The fans are necessary to keep the air flowing through (keeps bugs and mildew to a minimum). the fans are standard 120mm computer fans that run on 12v. to power them, i bought a power supply from radio shack and hooked it up to the fans (be sure to get the polarity right - fans won't run if you have the +12 on the wrong side).
Some other interesting green house ideas using IKEA household items here.