I have a massive Steam library. I'm not rich or anything, but I purchase a lot of Humble Bundles and I'm a sucker for 50 cent games on sale. I've acquired over 300 games/DLCs and it's really challenging to keep everything organized in steam categories. I spent a long time with two windows open trying to categorize based on tags, genre and rating. I recently went through and setup all controller supported into it's own category. I switched computers and Steam's cloud system overwrote it. I eventually gave up and just let things go wild in my library.
Happiness returned when I stumbled upon Depressurizer! Depressurizer provides you an interface for managing your Steam categories very quickly. I was able to auto-categorize based on User Tags, Genre, Store Flags, Ratings and a ton of other fields. Depressurizer will write to your Steam directory, but also export it to a physical file in case Steam forgets all your hard work. I've stored my app in my Dropbox folder so I can work on my library on my Surface and my main setup.
This app is a must if you have more than 50 games in your Steam library. Plus, it's an open source .net project! I may just contribute to it in the future :)