[[ My 2008 iMac won't boot up past the pinwheel spinning. Is there a way to use another computer (I have a Macbook and a mini) and simply use this old iMac as a monitor? Or is that iMac ready for the trash pile? ]]
Can't be used as a monitor for another Mac. Simply impossible on that vintage of iMac.
However -- even if the internal drive is bad, you can still connect an EXTERNAL drive via firewire, and boot and run the old iMac that way.
Boot times will be slower, but once up-and-running you probably won't notice much of a difference at all.
This assumes that only the internal drive has failed, and that all the other internal hardware is still ok.
If you don't want to do this, your realistic options are:
1. get the internal drive replaced
or
2. start looking for a new iMac (or other Mac).