The ones running in or [url=http://www.scummvm.org/]ScummVM can be run in the Mac version of either program, provided you find a Windows machine to install the games on then copy the installed files to the mac in some way (USB stick, CD-R disc, etc).
Now to find a list of which games use either DOSbox or ScummVM, but hey, if you have either a Windows box or an Intel Mac with Wine (or VMWare/Parallels), you can install the games you already have bought and check for yourself.

Can you port over PC games to run on the Mac? Not that I have one but if you could I would consider one in the future.

post over?

*port

ASPYR does just that: http://tinyurl.com/cc5c7n
Usually the titles seem to lag behind the original PC release though. I guess that's a given as they're ports.
But as far as a user doing this, it wasn't feasible to try when Macs were using PowerPC processors and PCs were using Intel compatibles. Others can probably say more to this, but the compiled binaries were not compatible with another system without some sort of emulation.

About correct.
Aspyr, MacSoft and others are contracted by the original developer (or publisher?) to port a game to Mac OS, when the developer isn't doing it themselves (Blizzard and id are two of the few who do develop both versions in-house). Though it seems nowadays some developers just slap on Cider to the windows version and call it a "port" (Spore is a prime example, and I bet many of EA's recent titles share the same lazyness) though running a Windows application through a modified Wine isn't porting (it's not emulation though, since there's no faked hardware, just the Windows functions are "translated" into Mac OS functions at runtime, but since everything is translated while it's running, hardware requirements will always be higher).