Just to demonstrate an idea I have for a new feature in Crystal. When you use composite as well as the blending effects of Crystal, the result can be ugly when you see the background over another window through transparent borders.
If your wallpaper is set to a frosty noise pattern, or some other small random/repeating pattern, then the effect can be quite nice. However, these patterns don't make good backdrops.
What I suggest for crystal is the option to set the background to something other than the current wallpaper, so you can have the best of both worlds.
Short of Crystal using the actual composite extention, I think this is the best current solution for transparent eye candy.
Someone would have to make some nice backgrounds though, my noise one sucks :)
edit: I meant to focus the Gimp tip of the day window as it looked nicest in blue, but I forgot. Can't be bothered to do it again :)
Quick link to Crystal: http://kde-look.org/content/show.php?content=13969
Afterthought: This would also allow you to apply fancy effects to your wallpaper in a second file, and have it show up through the crystal borders, expanding on crystal's built in effets.