I would like to create a meta box where the user can add or remove a simple input field that holds an image url on posts.
I personally now use on every project Advanced Custom Fields (http://advancedcustomfields.com/), it is well-maintained, easy to manage, extremely powerful and ever-evolving in terms of features. It has a free and a Pro version (right now the plugin creator is moving towards a new version where there is only Pro, not single Add-Ons). Your repeater field would be in the Pro version.
However, there seem to be other solutions plugin-based, like http://simple-fields.com/ which I though haven't tested, but it has a repeater field option as well and seems to be free.