I have a user who wanted to be able to put links to external sites in the top "main menu" bar. So I've created a plug-in that takes any links from a link category called "Main Menu" and puts them alphabetically in the main menu.

More info: Main Menu Plug-In

It (as far as I can tell, I just took the code from links.php) honors viewing restrictions that editors place on the links.

(If anyone knows of a better plug-in to put links from the Links into the main menu area, please, let me know! I'd like it to have an interface that allows choosing different mechanisms for choosing the links to be displayed and their order. I'm not sure I'm going to have the time to build that.)