Those who own but do not practice their patents are sometimes referred to negatively as "trolls" or "patent trolls," especially when they assert their patents against companies who are alleged to be practicing the inventions without a license. Well-known (in some circles) firms that are often identified by others as patent trolls include Acacia Technologies and Intellectual Ventures.
The term was coined in the 1990s. A Wikipedia discussion of the term's origins can be found here. A more, but not completely neutral term is Non Practicing Entity, or NPE.