Programmatically Filtering Items Displayed in a List

If I had a list page full of items, how easy would it be to, say, filter
those items according to a value passed in the querystring or something
(without creating a whole different view).

For example, lets say I have a number of categories, with current values
category according to a passed value?

Basically the idea is to have one "view" per category, without having to
create a new view each time I add a new category.