On-Screen Ruler - lightweight, free, semi-transparent ruler
Magnifixer - Magnifixer is a screen magnifier utility.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
gpick - A color picker and color scheme creation tool.
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.
gcolor2 - GTK+2 Color Selector