I started with Prototype. Not sure if it came out before or after the first jQuery, but it was the cool kid for a while. Then the world realized it was a disaster and started moving to jQuery...
Too bad Magento devs missed that newsflash. Nowadays it still bundles Prototype.js, and every theme and 50% third-party modules you install carry their own copies of jQuery, all at different versions, placed in random locations, and embedded into the page differently.