Javascript Tooltips Framework are so useful in informing users when needed without the need for a design challenge as they work well with any layout.
Opentip is a beautiful JavaScript tooltips framework that can work with jQuery, Prototype or standalone.
It creates the tooltips with HTML5 canvas so that any creative designs are possible and tooltips are rendered almost equally in all browsers.
There is feature for calling content into tooltips using Ajax, positioning them wherever you wanted or grouping the tooltips(so that only one tooltip of the same group can stay open).
A nice and unique feature is stems (the little pointers) which auto-arranges themselves into any direction.
Opentip has multiple, good-looking styles and new ones can be created very easily.
Website: http://www.opentip.org/