Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Selenium - Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that.
Invision - Prototyping and collaboration for design teams
Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
Zeplin - Collaboration app for UI designers & frontend developers
Browsershots - Browsershots makes screenshots of your web design in different browsers.