It is easy to use, has enough features for most users, and is free. Hard to beat that. You can use it to take screen captures, annotate images, and it integrates with Evernote.
Teachers can use it to capture and annotate images for students, professional development, e-portfolios, etc. and students can use it for class projects.
It is available for iOS, Android, Mac, Windows, and Windows 8.
Learn more at the source page below.
Source: Evernote Skitch website