It depends on what sort of money you want to spend and how serious you are about recording. ksv's right, something like the Griffin is fine for odd stuff but if you have a decent mic (as opposed to a nasty computery one) then you'll probably need a pre-amp or an interface with a mic pre-amp.
Is it an iMac or PowerMac you have? If its a PowerMac then you've got the option of PCI cards as well as USB/Firewire interfaces. I've got an M-Audio Delta/44 PCI card (4 in, 4 out) and my flatmate has an emagic USB interface with 6 in/2 out. Interfaces like the emagic or the Digidesign MBox are good (especially the MBox, it has Focusrite pre-amps) but I'd try and go for a good PCI card or an interface running on Firewire or USB 2. There's a new M-Audio firewire setup with mic pre-amps, S/PDIF etc etc for about $400 I think, looks nice. The Delta I've got it is about £200 (I think, could be more) although you'd need a pre-amp or a desk with something like that.
If you're multitracking stuff then Cubase SX is very good, also if you're running os9 then there's Pro Tools Free. The MBox comes with PT LE, not sure if 6 is shipping with it yet. Logic is also good.