Improve website performance by caching widget and menu output in WordPress transients.
This is a very simple but effective plugin. Most of the time you don't need your widgets to be updated on every page load, so caching them makes a lot of sense.
The code is nice and lean, so it's easy to see how this thing works. Since it uses a transient, you still get a performance boost even when on shared hosting, which is a nice addition.
EDIT: Addition of menus now makes it doubly awesome :)