On-Screen Ruler - lightweight, free, semi-transparent ruler
CubeTape - Mobile dimensioner Cubetape is a portable volumetric weight calculator. Dimensional volumetric weight for all freight types. Integrates with existing applications.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Meazure - What are the dimensions of that image? How big is this window?
Parcelhub Shipping - Multi-carrier parcel shipping solution
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.