WhatsThis Tooltip Plugin.
This plugin works well but is limited to a single icon to be used throughout the site, although being able to use any symbol for the icon is nice. There are also a decent amount of icon colors to choose from. The instructions and settings are pretty straightforward. The tooltip responds well and displays clean.
This could easily be a 5-star plugin if it was multi-purpose (ability to insert a different icon where needed, eg. info, warning, help, etc.). Also being able to style the tooltip box from the settings panel would be handy.
Thanks for a good plugin!