I've been hooked since I found out about how much you can do with black and white layers and blending modes in photoshop, and there's extremely few photos I don't use it on, as I think it can give amazing effects.
When I say black & white layers, you might think that this is just a way to convert your pictures to black and white, but with different blending modes you can get completely different results. Black & white layers set to different blending modes can give more vibrant colors, more gloomy colors, more contrast, lighter pictures, darker pictures and generally make pictures completely different, both if you want it in color or b&w as an end result.
This "tutorial" won't tell you exactly what to do to get a desired result, but give you an idea of the possibilities with black & white layers and challenge you to test it out yourself.
If you have CS3, you can choose black & white layer under adjustment layers. (the circle that is half white and half black, at the bottom of the layers palette), if not you can use channel mixer and check the box that says monochrome.
It's more or less the same, but you have a little bit more possibilites with the black & white, as you can add a tint, and preset "filters" to get a different black and white tones which can give pretty different result. With the channel mixer you can also alter the red, green and blue channels, which can give the same results as the preset filters in b&w, but in the beginning I will advice you to leave it as it is and just try out the blending modes.
1. Go to the adsjustment layers and pick either b&w or channel mixer, depends on what version of photoshop you are using. Leave everything as it is (except check the monochrome box if you use channel mixer) and click OK.
The picture will now be in black and white.
2. At the top of the layers palette where it says normal (next to the opacity control) you can change the blending modes. Test out the different blending modes and see what happens.
Some of the blending modes will alter your picture drastically and will look extremely over-edited, while others will make just small changes or not alter your picture at all.
3. Try changing the opacity (top of the layers palette, next to the blending modes) to around 50%, test out the different blending modes again.
Notice that some of the blending modes that altered you picture drastically at 100% opacity doesn't look as bad anymore and can actually give some cool effects.
4. Choose one of the blending modes that you liked, and play a little bit around with the opacity to get the effect "stronger" or "weaker" and see how it changes, leave it at an opacity level that you think looks good. Use the eye next to the layer you added, on the left side, and click on it to toggle it on and off to see the difference.
5. If you think some parts of the picture looks better without the layer, and other parts looks better with it click on the white box, which is called a layer mask, next to the name of the layer. Use a soft black brush (if you click d on the keyboard the colors on the brush will become default white/black. x will make you easily switch the foreground the background color) and paint on the parts of the picture you don't want to be affected by the layer.
Now you know the basic of the use of b&w adjustment layers and blending modes, and it's just to play around (I usually don't know what to use before I start editing a picture, and just try out a lot of different things). If you think it's hard to just test around without really knowing what to do or what you want, here are some things I've found out that might get you started:
If you want more "gloomy" pictures, multiply is a good blending mode to use, especially on skies. Can often be too dark on 100% so try out different opacities. If it is opposite, that it doesn't get dark and gloomy enough, copy the layer and you double the effect.
Overlay or soft light on a low opacity is good to just bring out some more contrast in the picture.
The mix of multiply or darken and screen, overlay or soft light might give similar results to HDR-pictures
If you want lighter pictures, screen and overlay can be a good combination. Often becomes too light and burned out on high opacities.
Often alter one part of the picture at the time, the sky might not look good with the same blending modes as the rest of the picture and vice versa.
Have fun and experiment! That's at least how I get my best results.
And at last, here's some examples of differences I have got by using b&w adjustment layers set to different blending modes. I often start by making some more contrast by adjusting the levels, so to just show the effect I got by the b&w layers, the before picture has all the other alterations (mainly levels), except the b&w layers.
This became a bit longer than I had intended, but I hope you still have understood and learned something, and if you have any questions just ask or come with other comments if you feel like it!