My current employer (Giva, Inc) has released another jQuery plug-in today called the Linkselect jQuery Plug-in. This plug-in converts a normal <select /> element into a component that can be highly stylized via CSS. While there are a number of similar plug-ins already, there are a several of key differences which we think make this unique:

Drop down menus are intelligently positioned to stay in the viewport

Specifically designed to work in a limited amount of real estate

Specifically designed to work well with elements aligned on the right edge of the viewport

Full keyboard support (emulates IE6's <select /> element)

element) Feature rich API (for updating value, replacing options, disabling elements, etc)

Many callback features to control behavior (on change, on init, on format, etc.)

Supports tabindex

We've put together an example page that demos many of the features and how to use the plug-in.