In reply to Toerag:
The DNT huts (which isn't all of the Jotunheimen ones but is most of them) are available to use out of season but you need to be a member with a key to let yourself in when they aren't in full-service mode (a quick look at ut.no suggests for most of them this is from around the middle of September).
You can join and pick up a key at the same time from a DNT shop in any of the big cities/towns.
Post edited at 11:42