I think you are thinking of a Acer Revo, a review of the older model is here:
http://www.trustedreviews.com/pcs/review/2009/06/05/Acer-Aspire-Revo-R3600—nVidia-ION-Nettop/p1
The newer model (3610) has a dual core processor, more memory, bigger hardrive and Windows 7 so pretty much eliminates the downsides in with the 3600, and neatly fits onto the back of your monitor to keep it small.
You can get them here:
http://www.simplyacer.com/product/id/655106
edit: the cheap one on ebayer, is a bargain however it comes with Linux which might be a problem, if you aren't great with computers. The one with Windows is £240 so personally I would get the £300 one which is ready to go.