However, there are still rooms for improvement and reasons to have standalone selector engines that are not part of any framework. Since selecting elements are the most common operational you would do, any improvement would increase the overall performance of your script.
Looking at recent development, there are definitely people believing the need for standalone selector. Here are 4 robust and small standalone Selector engines you can check out.
This is the new comer of selector engine which launched last week and does look very strong. It support CSS3 selector standard, small and fast. Their test shows that it is faster than most of the framework based selector out there.
Again all the standard CSS3 selector support, small file size plus from their test page, it does seems to be faster than Sizzle. (but I think it was testing against early version of Sizzle)
Another new selector engine released this year that implements CSS3 selector standard and is looking pretty strong in performance too.