I now have a second 'Must Have' plugin for iTunes, in addition to
Synergy; this one is
Octiv's 'Volume Logic.' VL is available as a Mac OS X iTunes plugin, a Windows iTunes plugin, a Windows WinAmp plugin, and a few other things.
The difference it makes in sound quality is absolutely stunning.
More seriously, when it arrives, I will happily give you a list of must-have software for your Mac. What'd you get? Mini? (And if you got a mini and did not get at least 512M of RAM, you're going to want to do that upgrade immediately.)