This is a list of low-fidelity, rapid prototyping tools.  This list is being added to (and changed) constantly.  

Most of these tools have a “digital” bent to them.  More non-digital prototyping tools can be found in the design toolkit section of this site.  That said, many digital tools can be used to prototype/test “physical” offers.  Intrigued??

Mobile Apps: http://popapp.in

Mobile Apps: http://www.invisionapp.com

Mobile Apps: http://www.proto.io

Mobile Apps: https://www.flinto.com

Mobile Apps: http://www.relativewave.com/form/

Apple Product Mock Ups: http://www.endloop.ca/imockups/

Creating App Icons: http://www.makeappicon.com

Smartphone Screens: http://mockuphone.com

Pop-up Stores: https://www.thestorefront.com

In-Context Screen Shots (mobile and web): http://placeit.breezi.com/

Web Mock Ups: http://keynotopia.com

Website Wireframes: http://balsamiq.com/products/mockups/

Website Wireframes: http://www.mockflow.com

Splash Pages: http://unsplash.com

Basic Landing Pages: http://launchrock.co

AB Testing: https://www.optimizely.com

A/B Testing: http://unbounce.com

Demoing Smartphone and Tablet Apps: http://www.airsquirrels.com/reflector/

IDEO Overview of Testing, Prototyping and Piloting: http://www.designkit.org/methods#filter

A Great Curated List of Tools from Steve Blank: http://steveblank.com/tools-and-blogs-for-entrepreneur