Funny, DH and I debate buying a power washer from time to time. The reason I don't want to buy one is that DH will power wash everything he can find on a regular basis. He's already obsessive about lawn care
A couple of rental things in addition to the list:
movers dollies- even if you're not in the process of moving.
Ditch witch
Sewer snake is also a good rental, though DH and I bought one to have on hand when our mainline backed up last year. Seems like those things always back up at the worst possible time. Growing up, I lived in a house where we often had to survive the weekend without a bathroom when the mainline clogged. There's a great deal of psychology involved with my insistence on owning one.
ETA- concrete mixer.