I really like Marvel app for REALLY rapid prototyping. Like, I can draw a really rough app IN the app on the bus.

Also great once the prototypes start to get more details as well.

It's NOT in beta. Go sign up now.

I've been experimenting with these types of tools over the past couple months. Here are my thoughts:

Flinto - Nice and simple way to see how your app feels. You can use basic animations to transition from screen to screen. It has some quirks, but I love it.

InVision - I found this to be a little more complex than Flinto, but not hard to use. They have some nice collaboration features if you need to work with other people. You can also make desktop prototypes, which you can't do in Flinto.

Pixate - I've only been playing with this for a few weeks but it seems like there is a lot of potential. It's really nice to see detailed animations and interactions before you jump into development. Pixate can do this. It's still VERY buggy, but its beta software so that is to be expected.

Keynote - I've used Keynote for the past couple years to mockup UI and animations. You would be surprised how effective it is.

Others There are others on my list to try: Framer, Quartz Composer, and Webflow. We are in an interesting time in design. The tools we have been using are good up till a certain point. These new tools are trying to solve a problem for us and it's fun to watch them battle it out :)

In my opinion, Invision is the best. You can easily use the prototypes within a user test as well. They actually have options specifically for user testing that allow hotspots to be hidden.

uxpin.com is good, its collaborative, very fast and has great libraries to use

Framer! Can't beat it.

There are a whole slew of them...

InVision — A lot of good features, good cust. service as well

Macaw.co — They tout themselves as flexible as an image editor, but writes semantic HTML and succinct CSS

Webflow.com — Another Drag and Drop style site for creating professional responsive websites

Adobe Muse — A turn-key solution for creating and hosting websites

Adobe Reflow — Dedicated app to create responsive websites, meant for “designers to share their responsive design intent

Google Web Designer — Originally meant for creating engaging HTML5 ads, GWD can also be used to create sites with the of ease of a Drag and Drop style editor

Others... Pixate - Mobile Prototyping(still in beta I think) Flinto - Mobile Prototyping Marvelapp.com – (also still in beta)

In love with Invision, and they've got a free level.

Also, Verify totally rules for small, bitesize user tests.

Proto.io Pixate

Weve used POP - Prototyping on Paper quite a bit for super-rapid, super-rough mobile prototyping

Here are some I use regularly, each serves me a different purpose:

• Axure ($289, $589)

• Keynote ($20.00)

• Xcode + Origami ($100)

Axure is really good for creating mobile/desktop prototypes and it supports responsive design as well as interaction design in fine detail. Keynote is good for setting up something simple to demonstrate an idea to clients. XCode and storyboarding + origami for native iOS prototyping.

Flinto for statics.