I love jQuery.
Its minimalism, quality and consistency allowed me do anything with a few lines of code (often just a single one).
It brought a clean syntax to an inconsistent language.
It offered an abstraction over incompatible browsers.
It was solid. It was freaking fast.
It made me feel like a powerful hacker.
And I loved that a 12 year old kid could teach people how to use it.
Its bright future was not obvious at the start. Around the time it was born, there were several other competing libraries: Dojo, Prototype, Mootools, YUI.
But jQuery kicked everyone’s ass.