Dominant #11 chords

The sharp-eleven chord is a dominant chord with a #11 extension.  A #11 is the same tone as an augmented 4, a flat 5 and can also be notated as +11.  For a B7#11, the notes would be:
B - root
D# - major-third
(F# - perfect-fifth, possibly omitted)
C - flat seventh
F - sharp eleventh extension

I've updated my chord inversions PDF to include fingerings for dom.7#11 .

What to play over a seven #11?

Lydian b7.  The most 'appropriate' scale to play is something called the Lydian b7.  The Lydian b7 scale is equivalent to the Mixolydian with a #4 (instead of Mixo's perfect 4). This gives you the tones of the root, 2, 3, #11, 5, 6, b7.

Altered Scale.  As with any altered dominant, the altered scale is a good choice.  The altered scale gives you the root, b9, #9, 3, #11, #5, and b7.  For B7#11 you can play B Altered.

Symmetrical Diminished.  The symmetrical diminished on get you the root, b9, #9, 3, #11, 5, 6, b7.  For B7#11, you can play B Sym-Dim. 

Whole Tone Scale.  Another interesting idea is play the whole tone scale.  This gives you the root, 9, 3, #11, #5, and b7.  For B7#11, play B Whole-Tone.

Melodic Minor at the 5th.   Playing the Melodic Minor starting on the 5th from the root will give you root, 9, 3, #11, 5, 6, b7.  For B7#11, you can play F# Melodic Minor.

Altered Scale at Tritone.  Playing the Altered scale based on the tritone of the root is the same as the Melodic Minor of the 5th -- it gives you the same chord tones.  For B7#11, you can play F Altered.