Excellent comments re Photoshop, Jerry. There're a lot of misunderstandings about what PS can and cannot do. Leaving aside compositing images (something else altogether) you can't make a bad image good with PS. But as Jerry states so well, what you can do is take a good photograph and make an excellent print/web image.
I hate to do the cliched Ansel Adams reference, but he is the photographer most familiar to the average person, so I'll do it anyway. Adams was a strong proponent of the fact that good photographs are made in the field, while good prints are made in the darkroom - from good photographs. The fundamentals haven't changed. Digital imaging simply means I don't have to breath chemicals to make the magic happen.
As to the original request, it'd help if some examples were provided. John West's comments here are right on target. Absent specific examples, I'd suggest there's a monitor problem. I haven't noticed a plethora of too-dark images posted here.
I run hardware calibrated displays here, but like John I'm often appalled at how my web images look on others' monitors when I check. There isn't much I can do about badly mis-calibrated displays.
Scott