It is easy to have a messy library on Steam, especialy since humble bundles stuff and discounts appears. Even if it is possible to manage game categories, it is too slow to proceed, after 5 games with new category it starts to be a pain.

Depressurizer is a tool that let you manage the categories. It can even download a prebuilt list of categories for you and assign it to your games automaticaly.

https://github.com/rallion/depressurizer