I used these pokeballs as prizes for a charity videogame marathon we had, and they were a HUGE success. They are easy to make, but if done right, they can look great!
Get all of your crochet gear together! For this, I primarily used size 5 hooks, but for balls that required larger yarn, I used size 6.
To make most of the balls, I followed this general pattern, inspired by Wolfdreamer:
1) Crochet 6 in magic circle
2) Inc in each stitch
3) Inc 1, sc 1
4) Inc 1, sc 2
5) Inc 1, sc 3
6) Inc 1, sc 4
7) Inc 1, sc 5
If the ball needs to be bigger, I would continue with inc 1, sc 6, but generally I didn't need to.
8) Sc in all
9) Sc in all; change color at end of round
10) sc in all
11) sc in all; change color at end of round
12) sc in all
13) sc in all
14) sc 5, dec 1
15) sc 4, dec 1
16) sc 3, dec 1
I would start stuffing here if you hadn't already started.
17) sc 2, dec 1
18) sc 1, dec 1
19) continue to dec and tie off.
20) sew in all loose ends of yarn
At this point, you can do any felt work or additions you need to. To make the center button of the ball, I followed this general pattern:
1) Crochet 4 in magic circle; change color when using a slip stitch to complete the circle
2) Inc 1 in all stitches
3) Finish with a slip stitch and tie off
Now you have a pokeball! :D