http://www.phpbb.com/community/viewtopi ... &t=2113726
I have no idea whether o not it is a good idea to use an in-dev mod.
http://www.phpbb.com/customise/db/mod/thanks_for_posts/
This one adds quite some visual clutter compared to how reddit shows upvotes as "(number)"
Edit: I will not add such a feature without being sure that it will not break the forum or make it hard to install updates to the latest phpbb version. I don't have the time to investigate unfortunately but please feel free to help out.