Balsamiq is the fastest and most focused low-fidelity wire framing tool in the industry.

Pros:

• It is fast, you can quickly sketch ideas fast
• Keeps you focused on basics, no fancy colors or design elements
• Perfect for low-fidelity wire framing
• Ability to convert design to PDF

Cons:

• Not great for mid-fidelity or high-fidelity mockups
• Some elements not easy to edit
• Limited collaborative functionalities
• Limited UI elements