I'm getting ready to sign-up for a Tiny Amigurumi craft swap and I'm SO excited! I love making miniatures - despite the fact that it cramps my hands and makes my eyes cross.
When I crochet my tiny amis I typically use embroidery floss or tatting thread with a 1.15 or 1.25mm crochet hook.
Here is an example of a miniature chicken I made recently using the Birds of a Feather pattern. The finished size was just over 1/2" tall.
It makes my fingers sore just looking at it again :).