There are four commonly used Dominant Scales, with two being unaltered, the other two altered. The choice betwen unaltered or altered can be summed up generally as: If a Dominant Chord is resolving up a fourth, use an altered dominant; if it is moving elsewhere, unaltered. Blues changes are a notable exception, since the first chord establishes the tonic, even though it resolves up a 4th in measure two.

The first scale is known as the Myxolydian Mode and is the basic choice for playing over Dominant chords. It looks like this:

Only use the fourth degree as a point of emphasis when you want the "sus" sound as it tends to obscure the sense of tension, which makes the resolution sound weak. Also, a passing tone is commonly added between the root and the b7, yelding what is known as the "Bebop Scale."

As with Major Scales, the problem of the 4th degree as a point of emphasis is solved by raising it a half step, yielding a Myxolydian #4. A brief rant: some folks call this a "Lydian b7". To me this is misleading, since a Lydian scale is a Major Scale not a Dominant Scale. End of rant. The Myxolydian #4 looks like this:

This scale can be used on any dominant chord regardless of context, with the raised 4th degree usable as a point of emphasis/rest. An easy way to get the characteristic sound of this scale is to play the major triad bulit on the 2nd scale degree (D triad on C7+4). The chord asociated with this scale is sometimes writen as C7b5. In actuality, the 5th is not flatted, the 4th is raised. Oops, I ranted again.

Once the 4th degree is raised, the only two area that can be altered are the 2nd (9th) and 6th (13th) degrees. This is done to provide more points of resolution when resolving up a fourth. When you alter the 2nd degree, it is raised and lowered, resulting in an 8-note scale known as the Diminished Scale. It is the scalar equivalent of a Dominant Chord with b9, #9, #11 and natural 13. It looks like this: