What is done with a Jedi's ashes after they are cremated?
After Qui-Gon Jinn was killed by Darth Maul in Star Wars Episode I: The Phantom Menace, his physical body was burned in a funeral pyre, presumably as part of Jedi funeral tradition.
I used Qui-Gon Jinn as the prime example as it is shown onscreen in the film. While he did appear to various people after his death, he apparently had not yet learned to return as a force ghost until long after his physical passing.
Note: Lest we digress into the discussion of why some Jedi disappear at death, I would not count Darth Vader as an example in this question. According to Steve Sansweet, and also Wookieepedia, Luke cremated only his father's armor with its prosthetics. His body had already gone on to become one with the Force. So tradition would probably not apply to the ashes of just his suit.