Apple is abandoning SCSI for FireWire on the high end and USB on the low end, and many of their machines have IDE for internal drives. All of these technologies are smaller and cheaper than on board SCSI. Sun is still using Ultra Wide SCSI in some of its servers, but is using IDE in its low end workstations, and Fibre channel on its very high end systems.

Apple does support scsi still, but in the form of a firewire to scsi adapter (very expensive, oops), or a PCI SCSI add on card. This is (obviously) only available in the tower systems with PCI slots.

The biggest problem with SCSI is that it requires proper termination, and even some hardware vendors can't get this right. IDE doesn't need termination (as much), because the bus length is shorter and there are fewer drives on the bus. USB and Firewire are prefered over scsi probably due to simpler bus connection, smaller cables, and hot plug capability, which is more appropriate for portable zip drives, scanners, cameras, etc. Meanwhile, much of the functionality of scsi has been subsumed into IDE.

SCSI has a much higher per-command overhead, which is balanced out with its superior ability to issue multiple simultaneous commands, which explains one of the reasons why scsi usually outperforms IDE in multiuser conditions, even when their bandwidths are similar. Firewire is fast, and beats older scsi, but newer scsi still is faster with its multiwire parallel bus.

SCSI is a good alternative when you need more than IDE or firewire can give, but still can't afford fibre channel.