The title attribute is an HTML standard element available across most tags. e.g.

<li><a title="Widgets listed by household function" href="/widgets/by-function.html">by Function</a></li>

I've used this attribute on some sites for usability; many browsers pop up a "tooltip" over the link with the more detailed description of what is on the other side.

I've been wondering if doing so is having a negative effect on my rankings (hidden text?) or if it has any effect at all on onsite or offsite keyword relevance calculations.

Does anyone know of any research done on this?