Miboru is an open source and local media organizer and image board. It's licensed under GPL, supports image and video previews, has a powerful tag system, and has a good looking UI.
Miboru is in early beta. It works, but might not work entirely as intended.
After you've dragged the .app file to the Applications folder, make sure to run xattr -d com.apple.quarantine /Applications/Miboru.app in a terminal. If you don't do this, it will say “Miboru.app” is damaged and can’t be opened. You should move it to the Bin..