Joining the Steam client beta sets you on a rollercoaster of emotions, most of which end in some form of frustration. So much as look at the client and it'll probably try to update. Sometimes, at least, that update is notable. For instance, the Steam Library last night received an update to make it easier to keep track of your owned and installed DLC.
Here's what it looks like:
Yes, Crusader Kings II has a lot of DLC.
The new info-box lists all owned DLC and tells you whether it's installed or not. To continue with the CK2 example, you might, for instance, realise that you have Sunset Invasion installed. If so, you can click the "Manage DLC" button and be taken to the DLC tab of the Properties menu—where you can uncheck to deactivate that ridiculous alt-history mess.
It's a nice little feature, and something that I hope is a sign of a new direction for the Steam Library view. For instance, I'd love to have a similar feature that lists Steam Workshop subscriptions. That would give me some reminder to deactivate all the dumb shit I'd loaded Skyrim full of.
To try it out for yourself, head into the Steam settings menu, go to the Account tab, and change you Beta Participation setting to Steam Beta Update.