Free Ruler is a screen ruler for OS X with many intuitive features. Great for Web developers, graphic designers and programmers.

Features:

• A horizontal and vertical ruler which can be used independently or simultaneously.
• Measurement in pixels only for now. Support for inches, picas, and centimeters is on the roadmap.
• Automatic alignment of rulers.
• The ability to group rulers and move them around as a unit.
• Shadowless rulers with customizable transparency.
• Interactive tick marks which follow the mouse location for more precise measurement.
• Independent resizing of rulers.
• Single key shortcuts for most ruler functions.

Note: While the software is classified as free, it is actually donationware. Please consider making a donation to help support development.