Is it possible to use an iMac's screen as a second monitor? Is it possible to use an iMac (early 2008 model) as a second monitor (note: not with a second monitor)?

I have a Windows 7 laptop (for work) and when working at home it would be nice if I could somehow connect my iMac to act as a second monitor.

Of course, the Mini DisplayPort on my iMac can probably not be used since that is for connecting an extra monitor to the iMac, but maybe there is some neat cross-platform application to do dual-display over USB?

Also, any solution for the same thing between a MacBook and an iMac would be nice to know as well.
I don't know your iMac spec, but look at Apple knowledge base


  With Target Display mode, you can use your iMac (27-inch, Mid 2010) or
  iMac (27-inch, Late 2009) with Mac OS X as an external display.
  Connect any computer or other device with a Mini DisplayPort to your
  27-inch iMac using a Mini DisplayPort to Mini DisplayPort cable or
  using a converter that converts other electrical, video, and audio
  protocols from another source device to Mini DisplayPort compliant
  signals.  (Cables and converters available separately.)