Everybody at work uses Balsamiq for wireframes and it's very fast and efficient once you get the hang of it.
However, I'm always kinda frustrated when using it because of the outdated UI and messy design of the elements within it. Of course, aesthetics are not essential here, since the tool is meant for creating wireframes, but still, I don't see why it wouldn't have a more pleasant, more modern UI (at least correctly align buttons vertically).
Here's a concept on how it could look like with a cleaner, lighter UI (I left the layout identical to the actual Mac app).
L or F like