How do you organize your library?

I have my favorites, (local) co-op games, a category for all of the AAA tier games I haven't played yet, a "now playing" category for games I'm currently working on that aren't quite as fancy as the prior category, and an online category for games that are primarily meant for online co-op. I still have a TON of organization to do, but the vast majority of games I care about/ I would jump to if I'm looking to play with a friend are categorized.

I also plan on making an "on the fence" category for bundled games I redeemed without actually considering whether I'd play them and some others. This is a really good tool for organization. I used it to find all the games in my library that supported local co-op. It'll probably have good ideas for you to use as category titles too, though it's based primarily on Steam user tags.