What useful custom jQuery selectors have you written?

Question

For me, one of the best, yet under-utilised feature of jQuery is the custom selector. I have a fairly trivial example of this, to pick out all text boxes that are empty:

Answer 1

I haven't written any, yet James Padolsey has a great collection of selector plug-ins (for elements in view, for external links, for elements with a specific .data property, etc)