https://www.pcgamer.com/you-can-buy-every-valve-game-on-steam-for-dollar15/

If you've got a hole in your Valve knowledge, then the Steam Summer Sale is the perfect opportunity to plug it. You can buy every single Valve-developed game for $15/£11.50 in the Valve Complete Pack, which is a 92% discount. And the price will adjust based on the games you have in your library: the more of the pack you already own, the bigger the discount on the remaining games.

I expect most people will have the vast majority of the 19 games on offer, but I'm sure there will be the odd game that's slipped through your net. Maybe you've played Half Life 2 but not Episodes 1 and 2, for example. Or perhaps you've played Half Life but not the remastered Half Life: Source.