How can I sort/organize my Steam game library?

Depressurizer is an application that will help you more easily manage game categories in a large Steam library.

You've got more than just categories in your library. You also have filters. You can filter the list by games you have installed, by games you have played recently and by games you marked as favorites. If you've played that game recently, you'll also find it in Steam's jumplist, or at the top of Steam's tray icon's context menu.

What you really need though is search; nothing really beats that. You don't even have to use Steam for that: if you told Steam to, all of your Steam games (except shortcuts) are added to the start menu. Want Team Fortress 2? Press the ⊞ Win button on your keyboard, type the first letters of the game, press Enter and you'll have it launched faster than any category or filter combo can.

Since your problem is deciding what to play, you order your games as follows:

• Favorites: Games you are currently playing, or games from your backlog that you intend to play soon. Don't be afraid to cut items from this if you stop playing. Your goal is to have as short a a list as possible when you need to pick a game to play.
• Completed: Games that were good, but you finished them, so they aren't candidates for play. Come back to this list if you want to replay an old game or need to uninstall something to make room.
• Rubbish: Games you will probably never play again. Things that were cheap in the Steam sale, or turned out to be really bad.
• No category: Everything else. Your backlog. Things you will play one day, maybe, but not today.