Balsamiq Mockups is a small graphical tool to sketch out user interfaces for websites and web / Desktop / mobile applications.
It is a clean tool that lets you get in the flow and stay focussed on structure rather than colors and icons. Mockups provides enough interactivity to replace prototypes most of the time, and makes it easy to collaborate and get feedback on your wireframes.
If you use Google Drive, Atlassian Confluence or Atlassian JIRA, you should try a plugin version of Mockups. The plugins integrate nicely with these platforms, whether you are hosting them yourselves or using them in the Cloud.
Mockups for Google Drive allows you to create, edit and share sketch-quality wireframes, all within Drive. You can even collaborate on your wireframes with others, in real time!