Sometimes when going through questions, I can see that someone has previously voted to close it.
However quite often I disagree with the close vote and I would like to vote for the question to stay open instead. However I can't do that.
Maybe adding it as an option to this popup:
But make it only show up when not doing a review task since the review tasks have their own way to vote to keep a question open.
Voting to keep a question open before it has even been closed is a little pre-emptive. Until a question receives five close votes or a moderator close vote there isn't any guarantee it will actually be closed, and, if a question sits in a state of being open but with close votes on it for a period of time after these votes have been cast, they expire automatically.
Additionally, for the people working the "Close Votes" review queue - there is the option to "Leave Open", which will count as a vote towards keeping the question open. This can only be accessed from within the review queue though. Three "Leave Open" votes in the review queue will take the question out of the Close Votes queue and start ageing the close votes as normal.
Finally, once a question has been closed, it isn't gone. At that point, users with a certain level of reputation as detailed on the privileges page will have the open to cast a reopen vote to reopen the question.
Cast Close and Reopen Votes