The Javascript UI library used by Craft.
Garnish is a Javascript library developed by Pixel & Tonic for Craft and other P&T software. It contains several classes that handle the heavy lifting behind common application UI elements, such as drag-and-drop, modal windows, etc.
Craft only ships with a compressed version of Garnish, but all of the source code is publicly available on GitHub: https://github.com/pixelandtonic/garnishjs