Very easy to customize the tooltip within your posts or pages.
I just had one issue and that is that the author didn't say where to find the functionality. But I found it after a while, you have to scroll down in a post or page and then you find the fields.
Great stuff. You can even replace the <a href="...</em> with <span class="whatever" so that the tooltip word doesn't become a real link(which is often annoying for non technical visitors.)
The class whatever then can style the tooltipped word to make clear it has a tip.
Excellent stuff!
http://wordpress.org/extend/plugins/skinnytip-tooltip-generator/