Prototip allows you to easily create both simple and complex tooltips using the Prototype javascript framework. If you also use Scriptaculous you can even add some nice effects to them.
Archived Links don't appear on your main links page, but you can still find them using tags or search. If the link is public, other people can still find it, too.
If you're posting many links, this lets you clean up your link list without deleting anything.
