There were several fairly well known libraries a couple of years ago, but as far as I can tell, jQuery won that battle decisively. It has almost 100% penetration in the projects I'm familiar with that use a general purpose JS library, and it seems to be the default for articles and blog posts I find on-line these days too.
Part of that success came from the ecosystem reaching critical mass. Many people know jQuery. Many plug-ins are available for jQuery. And the vast installed base usually means quick bug fixes even for recent browser compatibility problems.
If there is something out there that is better by a wide enough margin to justify giving up that established ecosystem, I'd love to hear about it. The ecosystem has plenty of imperfections, but I haven't come across anything yet that seems nearly comprehensive enough to displace the incumbent here.
David Mark's My Library[0] is the best DOM library available by a wide margin. It has supreme browser support, and it's "modular" with a custom builder. The code is so solid that many people have borrowed from it, including the jQuery project and myself.
I've also created a DOM library, but with a more limited feature set (it's only a few months old). See my submissions for details.
To be worth shifting away from the jQuery ecosystem to something new, the latter needs a compelling advantage and it needs to not have any deal-breaking disadvantages. Nothing I've come across so far myself seems to meet both criteria.
"jQuery was conceived specifically to address the differences in browsers, so we’re not going to abandon the essence of our philosophy and simply disregard the millions [still using] oldIE. Yet we also want to move ahead and take advantage of modern browsers, especially the growing mobile market."
The whole point of jQuery is that it's supposed to allow you to move ahead without losing backwards compatibility. Making this split -- especially with a version number, which implies a feature-freeze for 1.9 and new stuff only in 2.0 -- seems like they're just admitting defeat on their primary function.