Depressurizer is a program aimed at making it a bit easier to manage large Steam game libraries.
In addition to providing a way to quickly and easily modify games' assigned categories, it also lets you mark them as Favorites or as hidden.
Depressurizer can also autocategorize your games for you. Currently, it does so based on data from that games' Steam store pages. It can use genres, store categories (like "Single-Player" and "Steam Cloud") or tags.
It also saves your configuration information independently of Steam, providing an automatic backup in the event that Steam loses your configuration.