I was recently searching for free mocking tools and stumbled over Maqetta. It is open source and runs as a html5 app, so you can either run it locally or deploy it on a server. But it is more like a WYSIWYG editor than a simple wire-framing tool (although they include a sketching style). Not yet sure if this is good or bad, but the whole app looks promising.
Anyone ever tried it (and probably already has more insight than me)?