Algodoo

0!!=1
0.5!!=~square root of pi/2
1!!=1
1.5=1.18...
2!!=2
2.5=9.13700...
3!!=72­0
4!!=620,448,401,733,239,739,360,000
5!!=120!
6!!­=720!
7!!=5040!
8!!=40320!
9!!=362880!
10!!=362880­0!
11!!=39916800!
12!!=479001600!
13!!=6227020800
­
and so on.

(I forgot what 0.5!! and what 1.5!! was and forgot in comment 13!!=6227020800!

i have values up to 19!!, but i didnt release them yet

and also 0.5! is actually ((square root of pi)/2)!

0!!=1
0.5!!~0.951…
1!!=1
1.5!~1.187…
2!!=2
2.5­!!~9.13…
3!!=720
4!!~6.204 x 10^23
5!!~6.689 x 10^198
6!!~2.601 x 10^1746
7!!~4.529 x 10^16473
8!!~3.434 x 10^168186
9!!~1.609 x 10^1859933
these are all from calcES

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
1!!=1
2!!=­2
3!!=720
4!!=6.2044...*10^23
5!!=6.6895...*10^198­
6!!=2.6012...*10^1746
7!!=4.5292...*10^16474
8!!=­3.4343...*10^168186
9!!=1.6097...*10^1859933
10!!=­9.0519...10^22228103
xxxxxxxxxxxxxxxxxxxxxxxxxxxxx­xxxxxxxxxxxx
8!! is so funny!
I got these all from wolfram alpha