Select2 is a feature-rich jQuery based replacement for select boxes. It supports searching (quick option filtering via a search box), remote data sets, and infinite scrolling of results.
Select2 also supports multi-value select boxes and a minimum input setting which is useful for large remote datasets where short search terms are not very useful. Select2 have the ability to add choices automatically as the user is typing into the search field. This is especially convenient in the tagging usecase where the user can quickly enter a number of tags by separating them with a comma or a space
Look and feel of Select2 is based on the excellent Chosen library.