Ask HN: alternatives to Balsamiq Mockups?

I've been using Balsamiq Mockups for two years now but lately I've found myself becing increainsgly disatisfied with the product. The reason is that I lack certain interactivity features that would make life so such easier, e.g. having widgets you can actually interact with when in preview mode or be able to design a carousel in one single mockup file and not having to create one mockup for each content pane.

These types of interactivity aren't eye candy or "fun" things to have, but rather pretty crucial when rapidly creating prototypes and their nonexistence makes me groan each time I launch Mockups.

So I'm looking for alternatives to Balsamiq Mockups that has the same look and feel (lo-fi, faux paper prototyping) but embraces the things I've mentioned (Balsamiq seems to have taken a stand against this).