Some useful / fun robots and gadgets to use on google wave

Robots (Add to contact list and add to a new/existing wave)
Picsinwave - Replaces image URL in blip with that image
picsinwave@appspot.com   

Wave Alpha - Calculation from Wolfram Alpha
py-robot@appspot.com

Aunt Rosie - Real time translator in a wave
aunt-rosie@appspot.com

Hangman
wavehangman@appspot.com

DrWeather - weather for a city
shiny-sky@appspot.com

Gadgets (Add to a wave by pressing "add gadget by URL" button)
Play chess
http://gerculanum.appspot.com/gadgets/com.example.chessgadget.client.ChessGadget.gadget.xml

Napkin  - drawing board
http://my-wave-gadgets.appspot.com/wave/NapkinGadget.xml

Complete list available at http://wavety.com/google-wave-gadgets-and-robots/.