Good audio remastering application

If you're looking an audio editor DSP Quattro is an excellent program. Way cheaper than Peak and better IMHO. Support is great and updates are regular. For free you could also get Audacity. I guess it may also depend on what you mean by 'remastering'.

 

I take it you mean importing old analog masters from tape, tarting them up a bit and slapping them on CD?

If this is the case, then have a look at T-racks. It's a neat 24-bit capable app with EQ, limiting and compression and an output section.

It'll also run as a plug-in in Garageband (well, the AU version will).

It depends how serious you are about audio quality, I'd go 24-bit/96Khz into Protools and master with the Waves Diamond set, but I'm picky. You could record 16-bit/44.1Khz directly into Garageband and master with the plug-ins in GB if you weren't too bothered.