Hircine's ring. unlimited transformations.
One problem. The Ring of Hircine only works if you are currently a werewolf. AFAIK, if you've already cured yourself of lycanthropy, then it's no dice (in other words, it won't work as you won't get the ability required for transforming even if the ring is equipped, at least this has never worked for me, anyway). Also, I'd like to point out
Spoiler that it's really dumb how the werewolf comments from the guards trigger upon completion of a Companion's Guild quest instead of the dialogue being triggered by the player being a werewolf. I've heard that this is the case, but I cannot find the generic guard dialogue to verify it.