Roland V-Studio 20
I have just purchased one of these cheap (£99) from a local music shop largely with the motivation of testing how it works under Linux.
The answer is it works really well, as long as you have a recent kernel. The control surface also mostly works with Ardour 3.
If anyone is interested, the details are in my blog at http://majiksramblings.blogspot.co.uk/2014/01/roland-v-studio-20-with-linux.html
If the devs want to pinch the control surface definition and include it in future releases of Ardour, you are more than welcome.