steamsteam-trading-cards
When you collect trading cards and craft them into badges, you also increase your level.
Each level yields some rewards, like increased friends list size.
So at which levels do you unlock additional showcase slots to display various Steam related things in it, e.g. Achievements or Workshop?
As of the Steam Discovery Update on Sept. 23rd, 2014, Steam itself now shows you the DLC you own in the Store page without having to use browser extensions or look at the game properties in your library.
The blue markings on the left side of each DLC entry indicate that I own it already and if I hover over it, there will be a popout that shows that it's in my library.
Note that isn't perfect. In this particular example, Steam shows that I don't own Wonders of the Ancient World, even though I do, but I'm guessing that's a problem with that particular piece of DLC, as it's done that since release. However, Steam still won't let you buy anything you already own, so you're safe from double purchases.
As per this Steam forums thread, and this Community discussion:
By default, you have a limit of 250 people for your friends list and every level adds another 5 slots - which means that you get 50 slots per milestone (10, 20, etc).
In addition to that, linking a Facebook account adds another 50 slots - to a base of 300.
Best Answer
You unlock a showcase every ten levels, starting at level ten.