There are hundreds of Windows only games that are ported to the Mac by hobbyists. These are not professional or supported ports. When you download these ports... they are illegal copies of the game... so if you do not currently have a license for the game (normal Windows version) you are breaking the law. You can also download just wrappers for these games... which are basically a full port done, then the game files are taken out of the Mac app... you install your own game files from your purchased copy into the wrapper and then it works.
Wineskin is one such fully open source tool based on porting that anyone can use.
Cider is one made by Transgaming that is only used by large game companies for official ports. but many people hack them apart and make them run different games so you'll see a lot of Cider hacked games out there that are also not official ports.
Hobby porting can be anywhere from easy to very difficult, just on a game by game basis... but there are hundreds floating around the Internet that are done by hobbyists.
There is a community on the forus over at Portingteam.com that focus on just this.