Magnus of Smartelectronix
is apparently very excited by his new Mac Mini, because he says it's
inspired him to port his plugins to Mac OS X! To get you started, try
out NyquistEq, a gorgeous parametric EQ that can process treble right up to the Nyquist frequency — the highest frequency that can be represented in digital audio without aliasing. Check out his other plugins; Mac users should check out the donationware Ambience reverb, too, and PC users have plenty of other toys.

Free (donationware), Mac/Windows VST (no AU, sorry)

Geek alert: the technically-correct name for the Nyquist frequency is the Shannon-Nyquist frequency. Poor Shannon.