No matter if Google Wave is still not even in beta-phase, it has already been possible to develop extensions and addons. In general Google Wave differentiates between Robots and Gadgets. One on its own or both put together form a so called Google Wave extension. To be more precisely, a Robot is an application that runs on an external server (at the moment only on appspot.com) and works as a participant of a Wave (therefore you have to add a robot to your contacts list). In contrast to the Robot, a Gadget runs on the client and has to be installed. 

Since Google Wave offers a wide range of innovative project development, I'd like to introduce you into the Google Wave Robots. Instead of a huge tutorial on how to create one, I just want to give you a fully working Testrobot. As Google suggests, it's a real Eclipse IDE project. Make sure you've got a Google Wave account, Eclipse IDE + Java ee Package and Google Wave SDK addons installed.

A huge tutorial on how to set up all this stuff and creating your first robot will follow!

Download: Google wave Testrobot