Dramatic Effect
Going through some recent photos, I can across two shots which looked pretty hopeless when I took them (I was scouting out wedding photography locations with a bride and groom to be, and the weather that day was miserable).
But, I decided to have a play in old faithful Photoshop….
Here is one of the originals:
Now check out the difference after a little editing magic:

It completely changes the mood. Transforming the image is just part of the process. The other effect added here is called Diffuse Glow. It works well where you have light colours and you don’t mind if the detail in them is reduced.
You can find Diffuse Glow in Filters > Distort. I generally use similar settings, which are - Graininess of 5, Glow amount of 3, and Clear Amount of 19.
Here are another two landscapes to compare. First the coloured version of the cityscape:
Here is the exact same photograph, but with black and white (Channel Mixer, Monochrome is what I used for this one):

Diffuse Glow works well for portraits as well. Just be careful not to overdo it. I’ve done that before, with interesting results…