As the title suggests, I think it could be a great feature to notify people of significant sales on game they already own so they know now is a great opportunity to recommend games to friends. I think this could be a symbiotic, useful feature in the long term, improving the effectiveness of sales and furthering game's communities.

The exact implementation is of course up to your more experienced developers, but I imagine a small banner or perhaps a dollar sign or some other display next to the games in the library, or even an additional store tab with "games owned" where you see your games and the current discount amount listed next to them.

Just in case I am blind and there is already such a feature, feel free to point me to it :)