his prototyping shield is the best out there (well, we think so, at least), and now is even better with Version R3 - updated for the most compatibility with just about all the Arduinos! It works with UNO, Mega, Leonardo, NG, Diecimila, Duemilanove, and compatible Arduinos. Yun's and Arduino Ethernets have a chunky Ethernet jack that gets in the way of stacking, you can use the stacking headers included and it will work, just doesn't sit nice and flat.
Check out these awesome specifications:
Comes as a kit of parts so you can choose how you want to configure your shield. However its very easy to assemble even for beginning solderers. More information & instructions are available here.
Arduino and breadboards are not included!