It’s often much more efficient to access DOM properties directly, rather than using jQuery. The downside to that approach is that there are differences between the various browsers, so in some cases it introduces bigger problems than the extra performance overhead.
Rob Tarr ran some benchmarks to prove the claim that chaining jQuery selector methods is faster than placing them all in a single method call.
This Fiddle shows how to pass extra parameters to an event handler function in jQuery.
There’s a bug in jQuery 1.4.3 that causes document.ready() to fire twice if there’s an unhanded exception. You can avoid it by wrapping the code that is throwing the exception inside a try/catch block.
Ryan Cramer details some of the problems with select/multiple and a few good alternatives. It’s useful in certain circumstances, but unintuitive and error-prone. Instead, you can create a list of checkboxes inside an element with overflow:auto set, or use a jQuery plugin he developed.