TabNotes - Markdown notepad in your new tab
CONV. - Convert YouTube video to MP3, GIF or MP4
Typora - A minimal Markdown reading & writing app.
Downie - Video download for OS X. Downie supports at this moment over 1,000 different sites.
Markdown New Tab - Write notes in Markdown directly in the 'New Tab' page
Yout - YouTube to Mp3 Converter