On-Screen Ruler - lightweight, free, semi-transparent ruler
Repology - Multiple package repositories analyzer
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Anitya - A software release monitoring project.
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.
whohas - whohas is a command line tool that allows querying several package lists at once