(EDIT: I now see after having posted that it has already been proposed. But I think it is worth considering again.)
It is different from a previous feature request (about close votes).
The idea is to have a visual sign that you have upvoted/(downvoted) a given question in question lists and in particular in favorite lists.
One has already a visual clue that a question has been accepted and that one has favorited a question. I think that it would be useful, especially in the favorite list (where by definition you have only questions that you have favorited), to have a visual clue that you have voted this question up as well.
Your use of the site may vary, but I often favorite a question when I intend to go back to it later, as a kind of reminder. I do not always upvote the question or its answer since I may need time to read and assess it carefully.
It would among other things allow to go on recently changed questions for which you have still the possibility to upvote.
Another use on classical question lists is to allow to go quickly to potential interesting questions that you can upvote if it is deserved.
Going through old questions (especially with ambiguous titles), this is also quite handy to check if one has already been there or not.