OS X for Legacy Macs
Apple has chosen to limit the machines which are supported by Mac OS X, but XPostFacto can help get Mac OS X working on some older machines.
XPostFacto is an open-source application which you can download and try without registration. However, I would encourage you to become a registered user of XPostFacto. This will give you access to the tech support forum on this site. I do also answer e-mail, but I catch up on the tech support forum before I catch up on e-mail.
For more information, see the XPostFacto documentation.