It is very common to see people
use fingerspelling to show the concept of "WOW!" It is done by starting with
the "W" in the normal neutral fingerspelling location and then as you change
to an "O" you bring the hand closer to the body and then as you change to
the second "W" you throw the hand outward to the side.
Here's a way to sign "WOW" that shakes a loose "5" hand. This
version of "wow" can be used as a general
statement of intensity. "Oh boy!" "Oh my heck!"
WOW:







Here's sort of a pun. It is
a funny way to sign WOW. You use your
mouth to be the "o" in "wow!"
WOW (pun version)

You might also see variations where people spell W-O-W.






Notes:
There are even more ways to sign
"wow." One that I need to get around to posting uses a 'Y" handshape held in
front of the face with the thumb pointing toward the face and the pinkie
pointing frontward. The hand moves left an inch or two, back to the right an
inch or two, and left again using a very fast movement and an intense facial
expression.
[Need to post: "WOW" ("y"-hand version]