Photoshop CS2 has an HDR function, but from what i have seen it isnt as good as photomatix when it comes to output.  That is a personal view on it, but i beleive it enough that i am planning on buying photomatix as a plugin for my CS2 (i have it as a demo, but that plasters their watermark over the finished image).

The idea with HDR is you take a range of exposures, so there will be a very dark photo in one of them where the detail of the highlights can be taken by the camera, and the same with a light photo where the detail of the shadows can be seen.  The HDR process combines it all so that every part of the image has enough information and detail.

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography