Any trick to use my 2008 iMac (mini-DVI) as a secondary monitor for my MacBook Pro? I have a 2008 iMac (with mini-DVI and NO Thunderbolt) and I want to use it as a secondary monitor for my MacBook Pro.

The FAQ for Target Display Mode says that it's possible to do it with an 2009 or later iMac but not with the 2008 version I have.

Are there any tricks to do it by adding any kind of software, hardware, cables or adapters?
Yes - add a Mini DisplayPort port to your iMac. Should only take an engineer and a machine shop to accomplish.

In all seriousness, the mini-DVI port on your iMac is an output-only port, whereas the Thunderbolt/Mini DisplayPort ports also accept input. No amount of software, cables or dongles will change that.