"Everyone" uses jQuery. I've used jQuery, and liked it. jQuery is not a bad choice.
If you're not worried about a little extra fluff, though, take a look at Dojo. Dojo might come a little fatter, but it's got layers for lots of neat tricks. The syntax is not unlike jQuery, you can use DojoX libraries [lots of experimental shit, my fav is Dojox Storage], you can even write full Adobe AIR apps (linux/mac/windows clients) completely in Dojo.