Pencil is a standalone app or Firefox extension* that lets you create diagrams or UI mockups. It doesn't have collaboration/multi-user features. The Pencil Projects unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use.
- Works well, haven't used it 100% yet, but it looks and feels like it will work VERY well once I need it later on. No styling, but for wireframes, we shouldn't need styles.