I almost forgot about this fast, cheap, quick & dirty way to make a light table from stuff you can find at most thrift shops, yard sales or even in your garage:
_ on Flickr - Photo Sharing!
I love how simple it is and, well, it beat spending $400 from something not much better simply because it has the word "photography" printed on the label.
Of course you can also just build one out of PVC or CPVC pipe and fittings...