A fully open source (GPL) driver which supports all ?DisplayLink devices to date (all DL1xx devices) is in the mainline of the Linux 2.6.38 kernel, and can be compiled as a module for certain older kernels. It is called udlfb. It's still very much under development, but it's possible to write custom applications to it today (or to libdlo user mode library itself, bypassing udlfb). And it's also po