Skitch
Used by 24 peopleevernote.com ↗

“Skitch is another tool for red-lining and giving feedback, capturing and reporting bugs, or doing ultra-fast mockups.”
Chelsea Howe Interview, Creative Director (Electronic Arts) · via Uses This 
“Skitch is very handy for quickly flinging images up to the web, with minor edits - useful for previewing photos, or discussing screengrabs.”
Tom Armitage Interview, Game designer, technologist (Hide and Seek) · via Uses This 
“I use Skitch a lot for taking and sharing quick screenshots, as well as lightweight image cropping and resizing.”
Steven Frank Interview, Mac developer, co-founder of Panic · via Uses This