Fastest way to generate a random boolean

Small enhancement for the second option:

According to MSDN

public virtual double NextDouble()

returns

A double-precision floating point number greater than or equal to 0.0, and less than 1.0.

So if you want an evenly spread random bool you should use >= 0.5

rand.NextDouble() >= 0.5

Range 1: [0.0 ... 0.5[
Range 2: [0.5 ... 1.0[
|Range 1| = |Range 2|

The first option - rand.Next(2) executes behind the scenes the following code:

if (maxValue < 0)
{
    throw new ArgumentOutOfRangeException("maxValue",
        Environment.GetResourceString("ArgumentOutOfRange_MustBePositive", new object[] { "maxValue" }));
}
return (int) (this.Sample() * maxValue);

and for the second option - rand.NextDouble():

return this.Sample();

Since the first option contains maxValue validation, multiplication and casting, the second option is probably faster.