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.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
Invision - Prototyping and collaboration for design teams
Browsershots - Browsershots makes screenshots of your web design in different browsers.
Zeplin - Collaboration app for UI designers & frontend developers