It was back in 2009 that DisplayLink decided to provide Linux GPU support and be open-source friendly for their interesting USB-based graphics adapters and since then the support has only become more compelling. At first DisplayLink provided a simple Linux library, documentation, and then a frame-buffer and X.Org driver for the hardware.
In the three years since open-source DisplayLink support landed on Linux, their drivers have improved and opened up interesting possibilities like running nine USB-based displays on Linux.
With the Linux 2.6.39 kernel there were DRM infrastructure changes to support USB DRM drivers with hope that a DisplayLink driver would eventually land. Today there's an announcement by David Airlie that he has an open-source DisplayLink KMS driver to share with the world.
The "UDL" DRM driver supports DisplayLink devices and provides support for fb_defio, the KMS dumb interface, 24bpp via conversion to 16bpp, and hot unplug using new DRM core functionality. The DisplayLink devices can be hot unplugged while handling all of the necessary changes thanks to other improvements made within the Direct Rendering Manager. This new driver is licensed under the GNU GPLv2 by David Airlie.
Besides the USB DRM changes as part of David's five-patch series, there's also fbdev changes that these patches depend upon and are currently queued up in their "-next" tree. Hopefully this work will be on final approach in time for the Linux 3.4 kernel.
The patches and announcement can be found on the dri-devel mailing list.
As far as DisplayLink's current selection of products go, they have a variety of USB graphics adapters, USB projects, and USB displays available. Their technology is used in many USB display products from various vendors (HP, Targus, IOGEAR, etc).