Sound cards for Macs tend to be very high end now - such as the Apogee Symphony 64.
For lower end most people tend to use Firewire interfaces (such as the Apogee Duet or Ensemble), or for the cheap end of the market there are lots of USB devices by companies such as M-audio.
A few years ago I had a fairly cheap (but fairly good) M-audio PCI card, but it seems that with the move to PCI-X etc it's only the high end cards that have survived.