When Yoda said "there is another," I always figured it had to do with Darth Vader. The ending makes much more sense that way.
The counter-argument that I keep seeing is that Yoda was specifically referring to Leia. I don't think that this is necessarily true.
To me it seems like this: Yoda just didn't know who the other hope was. He felt a really strong force, and felt that it had some good in it. It doesn't mean he knew whom it belonged to. He was a Jedi master, not a prophet. Even if it were powerful enough to belong to a Skywalker, we can't really blame Yoda for not considering Vader a candidate. Seems to me that when he said "there is another," he meant exactly that, and nothing more: there is another hope.
Again: I don't think Yoda necessarily knew it was Vader. But I don't think he was sure about it being Leia either. Ben believes Yoda meant Leia, but there is a couple of reasons to think that he and Yoda differed on that. In response to "That boy is our last hope," Yoda's cryptic words are "No. There is another." It is an odd thing to say to Ben, who was well aware that Padme had two children. Yoda would have just said "His sister there always is." Ben's belief that Yoda meant Leia is mistaken, but understandable: Vader was quite a bad guy, and Yoda had reservations about Anakin from the get-go. It seems to me that the viewer is being intentionally misled in order to make Vader's change of heart that much more impactful.
The plot itself also suggests that Vader was the real other last hope. Leia was very helpful, but her help was a bit more general, not reflecting a "last hope" role. In addition, her help had very little to do with her being a Skywalker. She could have done all the stuff she did being a Dolan, Smith, or Schwartzbraum. Her being Luke's sister benefited Han Solo, but that's about it.
However, Darth Vader very much played the role of "another last hope." When Luke was beaten and the need for "another" became real, it was Vader's decision to turn from the dark side that made the difference; he very much delivered on his role as "another." And he did it precisely because he was Luke's father. The last hope did end up resting on a Skywalker - not because of his powers, but because of his love for his son. It makes a lot more sense than Leia's rather unfulfilled "last hope" role.