The Linux USB subsystem has grown from supporting only two different types of devices in the 2.2.7 kernel (mice and keyboards), to over 20 different types of devices in the 2.4 kernel. Linux currently supports almost all USB class devices (standard types of devices like keyboards, mice, modems, printers and speakers) and an ever-growing number of vendor-specific devices (such as USB to serial converters, digital cameras, Ethernet devices and MP3 players). For a full list of the different USB devices currently supported, see Resources.
The remaining kinds of USB devices that do not have support on Linux are almost all vendor-specific devices. Each vendor decides to implement a custom protocol to talk to their device, so a custom driver usually needs to be created. Some vendors are open with their USB protocols and help with the creation of Linux drivers, while others do not publish them, and developers are forced to reverse-engineer. See Resources for some links to handy reverse-engineering tools.
Because each different protocol causes a new driver to be created, I have written a generic USB driver skeleton, modelled after the pci-skeleton.c file in the kernel source tree upon which many PCI network drivers have been based. This USB skeleton can be found at drivers/usb/usb-skeleton.c in the kernel source tree. In this article I will walk through the basics of the skeleton driver, explaining the different pieces and what needs to be done to customize it to your specific device.