I imagine having to interact with Gmail is a necessity for most programmers. Gmail's control for associating labels to emails is monumentally effective UI behavior.
For those unfamiliar with the behavior, it's basically a button-initiated pop up panel (drop down UI effect) containing:
1) a text entry box 2) scrollable multi-checkbox list and 3) a submit button.
...all neatly and tightly stacked on top of each other. This panel allows the user to type into the search box to control the items of the scrollable checkbox list in real time. Once all desired items are selected, the submit button initiates the server action.
I imagine the most challenging aspect would be getting the the text box control to govern which items are in the scrollable checkboxlist without having to hit the server (or ajax at most).