jQuery: The "I don't have to learn JavaScript" audience, quick+easy to improve certain page interactions. Tons of plugins, but quality and customization options vary wildly.
Prototype: The "I'd like to roll my own components" audience, very Ruby-like, super-easy to do your own specialized UI stuff. Has a learning curve, however.
Dojo, YUI: The "I'm enterprisey and like to edit configuration files" audience. Componentized frameworks, which can do everything (but make tradeoffs for complexity).
And of course there are others, serving various niches.
Your choice depends: Site or app? And do you just want to skin some prefab components, but your site will look like thousands of others, or do you want to roll your own and do more complex stuff and stand out of the crowd?
It seems that jQuery has gained a lot of momentum lately. What are your thoughts on it and prototype? I'm just an 'end user' and try not to do too awfully much with JS, and they all seem like nice work to me... I'm just curious which way the wind's blowing.