Some of them implement fairly fancy JavaScript and CSS, but there is a simple way to plug in the default tooltip behaviour into your Apex reports.
When you edit your report column attributes, add the following code into the HTML expression.
<span title="#MY_TOOLTIP_COLUMN#">#MY_COLUMN#</span>
These attributes typically have decent help in regard to the sort of coding examples you can apply.
So this setup places the data from the selected column my_tooltip_column, and displays the value in my_column as you'd normally expect.
And is the display from Google Chrome:
Simple but effective - and flexible.
See a variation on this example here.