Opacity Magic

To quickly "throw together" some images, you don't need to do all that channel operations jive, you can get away with some swift cutting and pasting. All done with setting paste opacity.

The sample shown here is a couple of images pasted over a plasma background.

One hopes that this doesn't become an image cliche, so don't do this too often. :)

Accellerated Watermarks

Use white text, and mess with the opacity. Probably not the same as Federico's Watermark timp.