File Upload Library

I need a fairly simple upload/download service for my users to post and share files (docs, zips, jpgs, etc). There needs to be a registration/password system (no anonymous uploads). Each upload must be assigned to a category (photos, software, audio, documents, etc.) that the user will pick from a pull-down list. An admin back end would be needed to allow creation, editing, deleting users, categories, and files from the database. Size limiting of the files to be uploaded would be great, if possible. Users would NOT have to register or log in to see the categories/files. The listing of categories/files would be the main page. CSS design preferred.