It is no great shock that Mac users do not have access to as many games as PC users. Sometimes, it takes months (or even years) before a game is ported over to the Mac OS. Emulators, like Parallels, have attempted to sort this out by creating a virtual environment that tricks programs into running on a foreign platform. Although useful, these types of emulators are far from full-proof.
In order to make a more reliable platform, Transgaming has produced Cider, developed (or distilled) from the GNU/Wine project. Cider (like Winelib) is a wrapper allowing users to run programs and games natively on Intel Macs without any changes in the original source code. This means that new games can be released for Macs and PCs simultaneously without the developers having to make any alterations whatsoever!