I’ve had a steam account since 2003, so unsurprisingly I’ve accumulated a bunch of games over time thanks to summer sales, winter sales, humble bundles, …
As a way to give users a means to sort and categorize games, Steam introduced categories a while back. You can add arbitrary categories, and have a game in multiple categories, and categories are synced to your account so they appear on multiple devices. The downside is that you have to set categories for each game manually.

Here is where a nifty tools come in hand: Depressurizer (download)

It provides a user friendly interface for organizing your categories, making it easy to mass categories games. But it’s most useful feature is the ability to auto-categorize games. It can do so via various criteria like genres, Steam flags (e.g. “Single-Player” or “Steam Cloud”), Steam tags, Developer & Publisher info, How Long to Beat times, release year, and/or Steam review user scores.

Once automatically categorized it is easy to find games on your library that are Local co-op, or maybe you are feeling like playing a Dungeon Crawler game today, or just want to browse games you have with an Overwhelmingly Positive rating.

I use a unique prefix for the different auto-categories so it is easy to tell them apart in steam. It also doesn’t mess with existing categories, so it’s easy to manage both categories from depressurizer and your own manual ones.

And this is an example of how the end result looks like
in Steam