Using an iMac as a monitor for a mac mini
You could use the iMac to remotely connect to the mac mini over the network with apples remote desktop software. If you have a gigabit wired network between them then performance would probably be good enough to be usable for most tasks except 3d gaming or hd video I'd say. You cannot directly attach the iMac as an external display using a video cable however. Also, remote desktop access over wifi would probably not be ideal for everyday use

To actually perform this, just do the following:


In Finder, click the 'Go' menu option and then 'Connect to Server'
type in vnc://mac-name.local:5900/ or vnc://mac-ip-address:5900/


This will connect the two computers and you sign in with the credentials of the account on the remote Mac