I thought about suggesting this recently.
Nothing related to badges, but sometimes when I'm busy on other things I view only new questions on the first page, then (hours) later I might have more time and go further back than just first page. But with there now being a lot of new questions on the first page, going through the subsequent pages of questions I do hit the ones I've already seen previously.
While the visited link CSS code sorts it out to a degree and shows me which links I've clicked on, it would be useful to know at a glance on the list of questions which questions (and answers) I've voted on.
However having said that, I'm not sure how useful this would be.
The room on the question and answer up and downvote box is quite limited, so any additional info provided would also be limited, thus potentially reducing any benefit gained.
You'd only need one on question - up or down - as obviously can only vote one way there.
Then on answer, there could be up, down, or both. But this would mean you've voted on any answers in this question.
So it's not giving you a lot of info on the questions page due to limited data, but might be useful.
You can see from the picture I made above, it does look quite neat, however not sure how many people would care about this.
In fact, it would be much more useful to see if I've answered on a given question, than it would to know if I've voted on a given question or it's answers.
I'm sure (speculation) that more users would prefer to know when they've answered than if they've voted.
Both could be done, as per following image, but also see the tick which on it's own without if I've voted or not would be quite useful...
The tick would indicate you have given an answer in the question.