[arbol]

MooTools springs to mind

[Klonoar]

MooTools, Prototype, Dojo, YUI... what an era.

It sometimes feels like part of what we deal with now in the JS world is due to half the crowd not knowing what came before.

(Edit: to be clear, I think the other half is responsible for the advancements... so it probably evens out)

[Izkata]

One of the systems I currently work on has all five of those. Which one was in use was developer's choice for pretty much any given page or feature.