My Stinkweed Imp was in the graveyard, and I used Dredge to return it to my hand, and replayed it. Later in the game, the imp died a second time. Can I use Dredge on it a second time, or is Dredge a one time use?
Yes, you may use Dredge as many times as you want.
Whenever a card changes zones, it becomes a new object. It no longer remembers anything that it did, or was done to it in the previous zones. So even if an ability said "You may only activate this ability once per game", then the process of moving the card to your hand, then to the battlefield, and then to the graveyard would make the card "forget" that it had ever activated the ability in the first place.
Secondly, Dredge does not have the text "You may only activate this ability once", and in the absence of specific rules limiting how often you can use an ability, you can use that ability as many times as you want.