Being a former programmer, there was a time i had to choose a javascript library to use for my projects. At that time, there were many libraries avaliable out there that were impressive, like Dojo, Jquery, Mootools, Prototype etc etc.
After much consideration and research, the first library i chose to use was Mootools, due to its buzzing community and wide range of different impressive samples at that point of time. I did not regret my choice, as it gave me much joy and fun in developing many things i would not even dream of using HTML 2 years ago.
1+ year has passed since i made that choice, Mootools is still good as ever, but Jquery is catching up in the market share due to many reasons, one being its ease of learning the syntax. It is getting so popular that even Jquery is being incorporated into the ASP.Net Framework in Visual Studio. I think its time for myself to look seriously and deeply into Jquery so that i will not be outdated. =)
One interesting sample of what Jquery can do : http://kenneth.kufluk.com/google/js-mindmap/