Consider this Sanus if you want to mount it on the wall:
It's good for up to 80 pounds (the new 27" iMac is 30 pounds). It extends 3 1/2 to 20 inches and is very sturdy.
I installed a 32 inch LCD HDTV on it and I'm very pleased with it.
My local Best Buy has a sample setup on display.
|Douaumont Ossuary by Eric 54-BNF|
from Armistice Day
|Silhouette at sunset by Jill Hancock|
from Portrait Lens (around 80mm or equivalent - please check the full rules)