Now that Linux has fast system calls, good (and getting better) threading, and cheap context switches, it's possible to write device drivers that live in user space for whole new classes of devices. Of course, some device drivers (Xfree, in particular) have always run in user space, with a little bit of kernel support. With a little bit more kernel support (a way to set up and tear down DMA safely, and a generalised way to be informed of and control interrupts) almost any PCI bus-mastering device could have a user-mode device driver.