Originally posted by BartandLees:

I personally would find it helpful if Steam would say look at the games last played that are currently installed, and have a section to recommend uninstalling games in the client if say they haven't been played in two months.

Perhaps it doesn't even need to be that intrusive; a submenu under Library perhaps, right below downloads.

This would help clean up hard drive space really quickly for myself and I imagine tons of other people.