   In-Game GUIs, especially of the MMORPG kind...
  Just how exactly are GUIs like these made:Do they just wrap up Windows and use them, or do they write their own custom GUI system (ala Half-Life)?  The majority of them feature moveable windows with fairly obviously Windows-style controls (text input boxes, labels, control buttons, etc.). 
  They''re custom GUIs.Gamedev article showing you how-to in DX. 