Get instant feedback on mock-ups and ideas via 5 second Test, now a part of Usabilityhub, where you can test your designs and mockups on real people. Another great website is Usertesting.com.

In-depth feedback on prototypes and mock-ups via ConceptShare.com.

Use Keynotopia to create wireframes and prototypes via Powerpoint and Keynote

Launchrock or Kickofflabs are awesome to test your business case in a real-world context. Get inspiration on how to create a viral launch page and if people are interested in your idea you’ll end up with a significant mailing list (Bonus: use tools like Mailchimp to manage your mailing list).

Use Betali.st to prototypes ideas & get sign-ups from early adopters.

Don’t forget to A/B test you launch page with services like Optimizely.

Track traffic with Google Analytics, Inspectlet, Mixpanel and KISSmetrics, and check what parts of your website the users are actually looking at with Crazyegg‘s heat map tool.

Another way to test the viability of your project is by selling your prototype on sites like Craigslist or launch a crowdfunding campaign…. you can even do this before you have a prototype! For inspiration on how to hack your crowdfunding campaign read this blog post by Tim Ferriss & Ethan Mollick’s top tips on his website.