Very good question! We haven't documented this yet but we are working on it.
FEZ Domino is "pinout compatible" with Arduino and so you can use most of the available Arduino shields. We already have some on our website and more to come.
FEZ Mini is "pinout compatible" with Parallax BASIC STAMP2 and Inex POP168 and so any device (robot) made for these devices can now be programmed in C# with much easier interface.
Remember that these devices (Arduino, BS2, POP168) are "pinout compatible" with FEZ but FEZ has much more to offer, from file system and USB host/device to Visual C# for IDE and debugging capabilities....too many new feature to list here but they will be documented soon.