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.
I don't own six games in the pack (I've played some of them, but clearly not on Steam), including the original Left 4 Dead and Portal, and buying all six is only going to cost me £2.50 (around $3.30). And yes, I know most of the games are old and go for cheap often, but it's at least worth a glance to see what you can pick up. Click here to have a look.
Note that Garry's Mod is not included because it was only published, rather than developed, by Valve. Free-to-play games like Dota 2 and Team Fortress 2 are also not listed in the bundle, naturally.