Hardware Abstraction Layer (HAL)
Todo (Put here the generic, non-Matrox related, definition of a hardware abstraction layer --JoseFonseca)
The HAL module is also a binary provided by Matrox to provide extra functionality with its XFree86 drivers for the G400/450/550 chips. It provides support for dualhead and TV-out on G400 and DVI digital screen support on G450/550 cards. It also allows you to use their MergedFB implementation for dualheaded cards and their powerdesk configuration utility.
You can download it here. It's included with their Linux drivers.