I find that lowes and home depot have very useful personalized ads. I bough some concrete repair spackle, they saw it in the cart and suggested all of the other tools I needed (degreasers, cleaning brooms, chisel, sandpaper) that I would need as well. Which is extremely nice because the rest of their site sucks. Amazon also seems to have a nice book recommendation system.

Just as a contrast to most of the targeted ads which are mostly a waste of time.