I've been a fan of the Tiffen DFX Filters ever since version 1 and version 3 is definitely the best yet.
When using them to process some broken glass shots of Detroit (what else!), I ran across this interesting effect if you apply the x-ray filter twice
One application of the x-ray filter
Double application of the x-ray filter
The double filter almost looks like something you could actually see under natural lighting conditions.
3 hours ago