They are taking the world by storm!  You see them in Signs, Traffic Signals, Emergency Vehicles, and even Air Fresheners!  What am I talking about? LEDs!

There are thousands of LEDs out there to choose from, so what do I use to narrow them down?

Brightness:
LEDs are measured in Millicandella, and sometimes Lumens.  A LED that is to be used with others in daytime lighting should be at least 15,000mcd, or 8 lumens.  The brightness rating is taken from the center of the cone of light, and thrown into an equation with the viewing angle.  Thus a 10 degree viewing angle that is 130,000mcd is about the same brightness as a 20 degree viewing angle at 30,000mcd. DONT LET SELLERS FOOL YOU WITH SUPER HIGH BRIGHTNESS BUT SMALL VIEWING ANGLES!.

Size:
LEDs come in different shapes/sizes.  The most common is the 5mm circular LED.  5mm LEDs have a common current rating of 30ma Max.  Other variations are the 3mm and 10mm LEDs, as well as the square shaped "superflux" LEDs.  Real superflux LEDs are made by Lumiled (Phillips LED Lighting), and have a max current rating of around 70ma.  If they are 20ma, then they are not worthy of being called superflux.

Voltage:
The most common voltage for 5mm LEDs is:
2.0-2.8volts - Red/Amber
3.0-3.5volts - White/Blue/Green

12 Volt Wiring:
For the 5mm 30ma rated LEDs, the easiest way to wire groups of them is:
Amber/Red (~2.2volts each) - 4 in series with a 100ohm 1/2 watt resistor
                                             - 1 with a 580ohm 1/2 watt resistor
White/Blue/Green (~3.2 volts each) - 3 in series with a 100ohm 1/2 watt resistor
                                                       - 1 with a 470 ohm 1/2 watt resistor
You can also buy a "wall wart" power supply that is 110 volts AC to12 volts DC to power LEDs in your house. (try at your own risk, if the wrong cable is used serious injury or death can occur).

Coverage:
If the seller provides pictures of the LED's spot, look for a nice even coverage all the way to the edge.  Cheap LEDs will have noticeable hotspots and uneven color coverage.

I hope you feel somewhat enlightened on LEDs (pun intended).

One very reliable high quality seller that I would recommend is BestHongKong.  This is my #1 supplier.