If you are looking for a jQuery tooltip plugin to use for your project, http://craigsworks.com/projects/qtip/ is the right one for you.
It gives full flexibility in customizing the appearance and the behavior. Also the default settings are good, so with just couple of lines, you can put this plugin to real use in your project.
Another important aspect is its working fine in both IE8, Firefox and Chrome. The tooltip shows with rounded borders in all the browsers, and its achieving the rounded borders without images, which is cool.