If you are seriously considering downloading Half-Life 2 through illegal means, do expect quite a fight from Valve Software. The following is a response from Gabe Newell when being addressed on the topic of pirated HL2 copies:
Were running a bit of an experiment. Were keeping track of the accounts that do this and will be shutting them off. In other words, Valve has launched their very own "warez" version of their pride and joy whose purpose is to nail down those pirates out there. Thus, if you do not wish to plunk down $50 or so for one of the most riveting games of this year, the jokes on you, friend. If you have a warez version of Half-Life 2, Start uninstalling before you get yourself destroyed.
News source: CS Nation