NPC's - Not just the DM anymore.

Something I have played around with in the past and certainly plan to revisit in the future is introducing the players to an NPC who is roleplayed by a drop in NPC player in person or via Skype.

There are several ways to do this.  The most simple way by far is to recruit one additional player to the group who has a special enjoyment for roleplaying over power gaming or hack and slash under the understanding that 50% of their focus will be on roleplaying between two and perhaps four key / currently active NPC characters with which the party interacts regularly.

I would recommend that at least one of these major NPC's roleplayed by your friend is the major campaign villain appearing in the current landscape.  Certainly you can use this to good effect.  You can roleplay out brief conversations between the major villain and a crony or advisor.  Taking a few minutes out to describe some scene interesting to the unfolding story but which is out of character knowledge and allowing the players to watch this brief scene unfold can greatly add to the story experience of the players.  This sort of thing can really make for a more immersive story.

When I've done this in the past I've usually provided an NPC portrait, background, goals and resources to the person playing the NPC.  I also try to make sure the same person / player is available to roleplay that character at future game sessions.  I do try to generally limit the scope of time involved in this sort of thing.  Kozmodan the evil villain might appear in next week's session and so I contact a friend and get him to agree to come out and play both Kozmodan and a couple of other NPC's likely to interact with the players.  I give this helper a certain amount of creative room to try and play Kozmodan as a non idiot.  Kozmodan has a player now.  Kozmodan as a character wants his evil plans to work.  He will try to squash those that get in his way and often in very creative evil ways.

Obviously the DM needs to maintain some control over the game.  You might limit Kozmodan to capturing a character and later releasing the captive with a scar to remind them not to get in Kozmodan's way in the future...  Generally provide some limits that stop well short of free reign to arrange a TPK.  The main PC's are the heroes of the story after all and to a certain extent this should not degenerate into a one up between the PC players and the NPC player over who, as a player, is the bigger gamer guru.

Skype can be a creative way to bring in outside NPC talent for an hour or two during a game session.

It can be interesting for a special game night to bring in an entire crew of other gamers to roleplay the opposition.  Perhaps during a major battle or the biggest boss fight after weeks and months of adventuring.  Your usual group of five players comes to game night and when you give the signal three totally new faces, perhaps even decked out in sudo appropriate LARP gear come smirking out of the back office to take up the other half of the table to command the opposition forces.  Watch your player's eyeballs bug out when the DM is no longer rolling the attack rolls against them but Kozmodan and his couple of hench creatures are rolling attacks themselves.

This can certainly be used in a spartan fashion during a campaign.  Just here and there at points where you want the flavor and action to crescendo.  It is fairly work intensive.  Even more than the usual games which can be sufficient work already.  So I'm not sure I would recommend this shtick as something you pull out of your DM toolbox every week.