Now, you can't go hog-wild and solder up 50,000 LED's and expect your poor USB port to power them all.  I did much reading and pouring over the USB spec.  It's an intelligent disaster, if you ask me.  Personal opinions aside, any more than about a 100mA draw from a single USB port will cause the USB controller to disable the offending port until you remove the overload.  I used 20mA LED's, so 5 LED's was the limit.  You might get away with as much as 500mA out of an external self-powered USB hub, but don't plan on it (thanks, Mr. Tinkerer, for that!).

Best birthday present I ever stole from my brother.

Where is That Black Smoke Coming From?

Next, grab the USB cable.  Chop it in two and pick an end.  Strip off the outer sheath, and you'll encounter two layers of shielding.  The first is a layer of fine braided aluminum, and then some foil.  Remove those and you expose four wires.

The red and black are the power wires, and the thinner green and white wires are for data.  Clip the two blue and white, as we just want the power, not the data.  Traditionally, in a DC circuit, the red cable is the positive (+) and black cable is negative (-).  Now is a good time to check what's what with the volt-meter.  Plug in the USB cable to any USB port and check what's coming down the wires.  Label them.

From here we begin to build the lights.  Get your safety glasses and soldering iron.  I use a 700-degree tip on the iron and  700-degree rosin in the eye is not fun.  On a LED, the short pin is the cathode, which would like the negative black wire attached to it, thanks.  Long pin gets the red wire.  This is very important.  Don't get the wires crossed or the LED won't like you after that.  You won't get any light from it, and you might even fry it.

I first cut the black wire a little short, stripped and tinned it, then soldered the 75 Ohm resistor to it.  Tin the red wire, and solder the LED up, resistor gets short pin.  At $4 a pop, I didn't want to cut the green LED pins at all.  I cut several pieces of red and black wire for each new light.  Make sure you solder them together before the previous resistor on the black wire.  I used a clip to hold the parts in place.

Now that I think about it, you might find it much easier to twist all the wires together first, and then solder the resistor and LED onto the cable.  It will be much easier that way.