I know UI designers like using either Sketch or Figma — and after signing up to Figma this week, I can see why. But I’m going to give a shout-out to Balsamiq Wireframes (nee Balsamiq Mockups). I’ve found it to be a great tool for demonstrating a proposed layout for a UI without worrying too much about the details. It’s so quick to put together and the hand-drawn motif makes it clear that the drawn UI is just a concept.
I also love how they use text to represent visual content. An example is items in a table-view: instead of using the mouse to draw each row by hand, you configured it by effectively typing out a CSV with some Markdown. Use commas to separate the columns, use [ ] to indicate a column should have checkboxes. Such a great idea.