Trello allows up voting on cards via the Voting power-up (see Enabling power-ups). When the power-up is enabled, board members will see a "Vote" button on each card back under "Actions".
Adding / Removing a Vote
People can vote on cards by clicking the "Vote" button on the card back. The button will appear green if you have voted for the card. Clicking again will remove your vote. Hovering over a card and clicking "v" will also add or remove a vote.
To view a list of voters, click the vote tally box beneath the card title on the card back.
You can control who can vote (nobody, board members, members of the board's organization, or the public) by going to Menu > Settings > Power-ups > Voting > Settings > Who is allowed to vote on cards? Options will vary depending on board visibility and organization type.