Unlike the Mac Mini, the iMac isn't as popular for modding. However, I have found some notable iMac mods. Here is a list of the best and coolest iMac Mods.
Black iMac Case Mod
A simple, yet clean and elegant iMac case mod. The original iMac G4 (originally in a gloss white colour) has been given a semi-gloss black colour finish. The iMac G4 was entirely disassembled and each individual piece has been sprayed in satin black. For more photos and step-by-step details, you can see the whole project here.
iMac Vintage Steampunk Mod
Now here is an iMac you don't see every day! It's is a cool vintage iMac mod by Old Time Computer. The entire case of the iMac is hand crafted with Oak wood that gives it a nice vintage Steampunk look, it also features a full custom keyboard-mouse combo, which looks like a vintage typewriter. Check out their website for more iMac mods.
iMac eSATA Mod
Since we won't see Apple introducing eSATA to their Mac's in the foreseeable future, the guys at OWC have found a way to add an eSATA for the 2010 iMac. Now your iMac can have an external hard drive at ultra-fast transfer speeds. A very clever iMac mod.
iMac G4 iPad Stand
Here's quite a unique iMac mod, turning an iMac to a handy iPad stand. By removing the monitor on the iMac G4 and attaching a bracket to hold the iPad instead. This lets you take advantage of the iMac G4's swivel feature, being able to position the screen to suit your needs. This clever mod, coupled with a bluetooth keyboard, gives the iPad a desktop look and feel.