Is it possible to login as / switch to a specific user using their password hash from the DB?
There's Masquerade.

Masquerade adds some items to the Navigation menu to allow quick switching and switching back. It also creates a "Masquerade" block for quick switching which can be added to a sidebar or elsewhere on your site. It is visible only to those users with permission to masquerade.

Another module which accomplishes something similar is Role Switcher.

--a different Dan H