Trello. My wife uses it as well. It sounds silly at first to use something so engineering/workplace minded in a relationship, but it actually works really well for us because my wife and I have two very different styles of task management (and as newlyweds, we needed to quickly figure out a workable system!)

Her strategy is to break all tasks into little manageable chunks and complete them over a period of weeks. My strategy is to sort all tasks into a giant priority queue and then complete each task in one non-stop session as late as possible but early enough that is still gets done. For those who are deadline-averse, my strategy is clearly anxiety inducing, so Trello allows to us pick dates for each other that we would like tasks to be completed by. So far, this has eliminated all difficulties and confusion involved in getting stuff done.

The next step is establishing a shared calendar so we can track events, but I haven't found one I like yet.