Every so often I find a 'new' feature in Photoshop. Which is odd - since the feature in question usually isn't new and I've been using the program rather heavily for the better part of 10 years.
The most recent discovery is the Shadow/Highlights adjustment. It became a part of Photoshop in CS(1) - as of writing CS5 is almost due - and is also available in the more recent Elements versions.
While there are many ways to adjust pictures values in this way - brightness/contrast, levels, dodge/burn, exposure - Shadow/Highlights is both easier to use successfully and more dramatic in effect.
Quite simply, it takes the extreme areas of an image (without having to pre-select them) and drags them back to a normalized value. Which is probably best shown with pictures.
Highlights 50% + Shadows 50%
The only slightly confusing bit is that you're reducing the appearance of the said regions - so as shown above 50% Shadows makes the image brighter overall and 100% Shadows is even less 'dark'.
There's also an "Show More Options" version of the toolbar (at least in the full Photoshop) that will let you tweak things even more. Which probably wouldn't be a bad idea - as nice as the defaults are at bringing out 'hidden' detail - they tend to make the pictures more than a little 'flat'.
Original image inside red circles, 50% Shadow outside - Note how the water doesn't change much
More dramatic example - Shadow 100% on the right side of the image