Tooltips are so useful in informing users when needed without the need for a design challenge as they work well with any layout.
It creates the tooltips with HTML5 canvas so that any creative designs are possible and they are rendered almost equally in all browsers.
There is support for calling content into tooltips with Ajax, positioning them wherever wanted or grouping them (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.
Opentips has multiple, good-looking styles and new ones can be created very easily.