I'm just started experimenting with LEDs, and they seem to work best as additional lighting instead of being the only light source. I bought some 10mm high brightness blue LEDs from Radio Shack (cost: $2.80 and are around 640 to 660nm... can't remember the exact spectrum) and wired them to a computer power supply (they can take up to 3.8 volts, computer power suplpy has a 3.3v wire or I just read that you can hook them up to a cellphone charger, but will have to look into that.) and they seem to be very bright, but the angle is only 16 degrees, which make the light too concentrated... so I need to diffuse the light. I am using a 250W HPS light (which is in the yellow to red spectrum), so adding the blue will hopefully help them in the vegetative stage, then I want to switch to Red LEDs for the flowering stage.
Blue and red LEDs fall into the best light spectrum, they just don't seem to put out the lumens and heat unless you have enough of them. Since I am just starting with LEDs, I figure 3-5 10mm LEDs (blue or red) per plant should be enough to make a noticeable difference, maybe up to 10 for each plant if you are using only LED. The 10mm LEDs are fairly new and I have not seen any post of anyone try to use them to grow yet, but they can certainly throw some major light out. I hooked a blue staight to the power supply with no resistors and it was very bright at 15 feet! I threw two resistors on at about 180ohm which dimmed them a bit, but should extend their life to spec. I don't know how long they would last with no resistors on them, but that's something I will have to play with. I will post some pics of the setup once I make a better fixture for the LEDs... Hope this info was able to help, would love to hear if anyone else is using the 10mm LEDs.