USB Output Voltage

isaacmc,
ELECTRICAL SPECIFICATIONS
USB transfers signal and power over a four wire cable, shown in
below. The signaling occurs over two wires and point-to-point
segments. The signals on each segment are differentially driven into
a cable of 90W intrinsic impedance. The differential receiver features
input sensitivity of at least 200 mV and sufficient common mode
rejection.
There are two modes of signaling. The USB full speed signaling bit
rate is 12 Mbs. A limited capability low speed signaling mode is also
defined at 1.5 Mbs. The low speed method relies on less EMI
protection. Both modes can be simultaneously supported in the
same USB system by mode switching between transfers in a device
transparent manner. The low speed mode is defined to support a
limited number of low bandwidth devices such as mice, since more
general use would degrade the bus utilization.
The clock is transmitted encoded along with the differential data.
The clock encoding scheme is NRZI with bit stuffing to ensure
adequate transitions. A SYNC field precedes each packet to allow
the receiver(s) to synchronize their bit recovery clocks
The cable also carries VBus and GND wires on each segment to
deliver power to devices. VBus is nominally +5 V at the source. USB
allows cable segments of variable lengths up to several meters by
choosing the appropriate conductor gauge to match the specified IR
drop and other attributes such as device power budget and cable
flexibility. In order to provide guaranteed input voltage levels and
proper termination impedance, biased terminations are used at each
end of the cable. The terminations also permit the detection of
attach and detach at each port and differentiate between full speed
and low speed devices.
http://www.semiconductors.philips.com/acrobat/various/USBSTAND.pdf
PL

The physical connector has 4 terminals.  The 4 terminals are:
1 - VBus - Red Wire  (Nominal 5V, spec requires 4.4-5.25 Steady state.  Transient can dip to 4.07)
2 - D-     - White Wire
3 - D+    - Green Wire
4 - GND  - Black Wire (return current)
Shell - Shield - Drain Wire (earth ground)

In a series A receptacle (the wider one), the connectors are numbered 1-4 from left to right when looking into the receptacle where the connectors are hanging down.

In a series B receptacle (the narrower squarish one), the connectors are numbered 1-4 in a counter-clockwise direction starting in the upper right hand corner when looking into the receptacle with the narrow side up.

Actually, you may want to keep the current under 100mA.  In suspend mode, the long-term average current draw is supposed to be under 500uA, but you are allowed to draw up to 100mA for up to 1 second.

There are a number of ways to limit the current you draw.  You can put a resistor in series with the bulb, or you can build a current source using a low power op-amp which can be used to power a bright LED.