In Photoshop, go to Edit > Fade… or Command-Shift F
Of millions tools you can use in Photoshop, Fade is my third most used tools for one simple reason: it will make your work look subtle and professional. When you does an one time change to your layer that you liked but felt that it was too strong and bold, you can get it to halfway between original one and changed one by just setting Fade opacity to 50% in Fade window pictured below.
The best example I can give you about the effectiveness of this tool is by comparing two same images, one’s original and other one’s sharpened (PS Filter). Sharpened one is too much to my eyes so I went for 50% to get details to stand out while not looking too abused:
It doesn’t have to be for pixel art only, it can be for anything such as digital photograph:
Or more extreme example using Texturizer (Burlap) Filter to create crumpled photograph paper:
Fade opacity is set to 30% and Mode is set to Overlay.
You can use Fade tool with almost every filters, adjustments, etc. Mode option found in Fade window is rarely useful for me since I prefer to do it via separate layers with layer mode on so I can change layer mode anytime I want to in the future instead of it being permanent.
One thing worth noting is that the usefulness of this tool will be less with upcoming Smart Layer in Photoshop CS3 but Fade will always have its place in Photoshop for a quick and dirty job.