Today Valve launched its Steam Labs Experiment 15, also known as the DLC Discovery Hub.
The feature that you can find here, does exactly what it says on the tin, it helps you find DLC for the games that you already own on Steam. All you need to do is to load it up while logged into your Steam account (or log in after you loaded the feature, I suppose), and you'll be greeted by a list of the games you have played recently and the DLC available for them.
You can also switch to listing your most played games first if you prefer that perspective.
It's certainly a handy little tool, even if we could say that Valve is certainly not doing this out of the goodness of its steamy heart. Obviously, DLCs are a massive business opportunity nowadays, and better discoverability means more sales.
That being said, especially for games that have a ton of DLCs, this may be a good way to notice whether you're missing something. Interestingly, the feature also tells you how many games you have, how many DLC you have purchased, and how many DLC for the games you own are still available. Believe me, that number can be slightly scary if you have a big library.
Since this is an experimental feature, it's very likely that it'll be improved further in the future, but it's definitely usable even in its current state.