1. You can customise the content creation wizard to add an additional step where users have to select a category.
2. If users are using CIFS to upload content, you can have a space for each category. Just by dropping the document into that space the category is assigned to the document. Depending on how many categories you have this can be a very complex solution. This is typical in EDRM implementations where you have a file plan that can be associated with one or more categories.