From Todd Ditchendorf:
Shapes is a simple, elegant Diagramming app for Mac OS X Snow Leopard. Shapes gives you all of the most important features you need in a Diagramming tool without all the extra cruft, and without breaking the bank. Shapes is great for Programmers and Web Designers looking for a simple tool for quickly designing Charts, laying out Wireframes, or visualizing Model Relationships. Shapes lets you get to work quickly by giving you just the basic tools you need in a slick, single-window, thoroughly Mac-native UI. Bonus: Shapes also offers Full-Screen Mode.