What add-on was this?

Augmented Steam

RaChart™ Enhancer (web.archive.org) / GitHub | Sighery

I use Steam Web Integration for that, it doesn't highlight but adds a cross, a tag, a heart, etc. to everything that links to a Steam store gamepage.

If you're talking about the tables in posts such as this that would be RaChart Enhancer, as NateSCC said. The ones you own are green, the ones you have wishlisted are blue and I think orange for not interested?