Here's a similar deal that made the rounds a couple years ago. I like it better cause there's no math:
Le Woogie

The crystal ball works because you will always come out with a number that is a multiple of 9, for example:
10 Gives you 9
11 Gives you 9
12 Gives you 9
13 Gives you 9
14 Gives you 9
15 Gives you 9
16 Gives you 9
17 Gives you 9
18 Gives you 9
19 Gives you 9
20 Gives you 18
21 Gives you 18
22 Gives you 18
23 Gives you 18
24 Gives you 18
25 Gives you 18
26 Gives you 18
27 Gives you 18
28 Gives you 18
29 Gives you 18
30 Gives you 27
31 Gives you 27
32 Gives you 27
33 Gives you 27
34 Gives you 27
35 Gives you 27
36 Gives you 27
37 Gives you 27
38 Gives you 27
39 Gives you 27

the number is always 9 * the first digit so, there are only 10 possible numbers for that particular problem.

So Ace, should I tell you how fast I figured it out or is my status as Biggest Geek still secure? :)

Later,
bbeck

Yeah...

It's just floor(n/10)*9. So the symbol at 9*k where k is any positive integer, is always the same.

If more rigor is desired:
x = first digit
y = second digit
(10x + y) - (x+y) = 9x
So answer is multiple of 9

Slightly different, but along the same lines, is Andy's Amazing Card Trick.

Another fun one is Petals Around the Rose