The 2 main wavelengths that I have seen for sale on the market are 850 nm and 940 nm. The difference between the two is pretty significant, so depending on what you will be using it for determines which wavelength you should use.
If you need to replace the IR led in your television remote, then you would want 940 nm. You cannot see any light at all being emitted from these. The only way to really tell if they are working is to look at the end of the led through a camera. You don't need a special night vision cam to be able to see if it lights up or not. Any camera will be able to see the light, just not well enough to be usable in complete darkness. This wavelength also tends to be a little more expensive.
Next is 850 nm. This is what most of us want to use with our night vision cameras. The only downside to this wavelength is that you can see a faint red glow at the end of the led while they are on. For what we use them for, that is definitely not a problem. The wavelength is brighter than the 940s and they are a little cheaper, and every little bit helps when you are on a budget. What I mean by brighter is that our night vision cameras can see this wavelength a lot better.
UV light looks like a purple/blue light.
WARNING: YOU SHOULD NEVER LOOK DIRECTLY AT UV LIGHT. ALWAYS WEAR PROPER EYE PROTECTION WHILE USING UV LIGHT. NEVER ALLOW KIDS TO PLAY WITH UV LIGHTS OF ANY TYPE.
If you have ever seen a blacklight, then you pretty much know what UV looks like. It is amazing what UV light can reveal. It can be used to check paper money to see if it is counterfeit, find leaks in the air conditioning system on your vehicle, search the carpets in your house for pet stains, allow you to hunt for scorpions by illuminating their bodies, check to see if the painter you hired put a thick enough coat of paint on your walls, or just play CSI.
UV lights used on paranormal investigations are being used in conjunction with Full Spectrum Cameras that can see this spectrum of light. Being at the opposite end of the light spectrum, UV gives us the ability to see things that we could not with IR light. Normally, if someone is using a camera that is Full Spectrum, they will illuminate the room with both UV and IR light at the same time.