Older iMacs had the option of mounting a VESA adapter plate, which would give you the ability to use any VESA standard mounting. I have one set up for my daughter with a 27" iMac she got in December of 2011.
"The Flat Display Mounting Interface (FDMI), also known as VESA Mounting Interface Standard (MIS) or colloquially as VESA mount, is a family of standards defined by the Video Electronics Standards Association for mounting flat panel monitors, TVs, and other displays to stands or wall mounts. It is implemented on most modern flat-panel monitors and TVs.
Apart from mounting the monitor, it can be used to attach a small enough PC to it.
The first standard in this family was introduced in 1997 and was originally called Flat Panel Monitor Physical Mounting Interface (FPMPMI), it corresponds to part D of the current standard."