In Linux there is also another SCSI access method via SCSI_IOCTL_SEND_COMMAND ioctl calls, which is deprecated. Special tools like 'scsiinfo' utilize it.
There are some other similar interfaces in use in the un*x world, but not available for Linux:
CAM (Common Access Method) developed by Future Domain and other SCSI vendors. Linux has little support for a SCSI CAM system yet (mainly for booting from hard disk). CAM even supports target mode, so one could disguise ones computer as a peripheral hardware device (e.g. for a small SCSI net).
ASPI (Advanced SCSI Programming Interface) developed by Adaptec. This is the de facto standard for MS-DOS machines.
There are other application interfaces from SCO(TM), NeXT(TM), Silicon Graphics(TM) and SUN(TM) as well.