Netscape 6 renders a link's Title attribute as a tooltip, in the same fashion that Explorer and Opera do. It shows as the little flyout that pops up when the cursor hovers over the link. Sort of like an alt tag for a link instead of an image, although things can get confusing when the link is also an image.
You can see the Title attribute at work on Jakob Nielsen's site [useit.com]. If you hover over his name at the top of the page, the Title "Author biography" pops out.
The Title attribute seems to be an under-utilized feature of HTML today. I think that the original idea was to include the title of the document being linked to, but any kind of explanation of the link's destination might increase usability in various situations.
I was just wondering if this could also help ranking. For instance, if Google indexes a link's Title, and it has good keyword content relevant to the linked page ...