Ranked in these QuestionsQuestion Ranking
Pro Easy to learn
There are easy to understand commands that really help to getting started quickly. The application itself is surprisingly simplistic with easy to learn shortcuts, but extremely powerful.
Pro Available on Windows, Linux, OS X
Binaries are available for Windows and OS X. On Linux it needs to be built from sources.
Pro Open source
Pro Good tutorials
Pro Extremely light-weight
The latest version 2.3 is ~6MB size, the previous one was ~1.4MB - yes, Megabyte.
Pro Useful integration of keyboard shortcuts
Pro Does not require installation
The single executable can simply be launched.