I just tested out Pointing Poker and it seems to do everything I need.
One person starts a session and sends the others a link. The votes get revealed to everyone at the same time when the last person votes or when someone clicks "show votes."
And what's really cool is that you can set custom values. My team uses a second deck of poker cards with team member names to figure out who should do something when nobody volunteers. I could set up a "human player" deck in pointing poker if it is needed.
(there's nothing like a snowstorm to cause you to figure out how to run a distributed sprint planning meeting on very little notice!)