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.
|Valley by the light of a blue moon by cjf2|
from Down in the Valley
|Lake Erie Stone Pier by yobbyt|
from Dock or Pier