Specifically, do PCs from previous campaigns find their way into your current game as NPCs?  Since I play in the Realms almost exclusively, it gives me a lot of extra NPCs to draw upon.

Usually, a couple of normal recurring NPCs are Titrenu, Scut's old Jester/Wild Mage (nickname - Titty.  His alignment is Chaotic, nothing else.  We took away the rest of his alignment when we realized just how crazy he is), Emalaithe (a witch that is actually a Countess in Cormyr), and Keiran Crownsilver (a Marquis in Cormyr, Moonshae born, heir to the Crownsilver family)

Nope. Never had happened yet that two campaigns are set in the same universe at roughly the same place...

Yea, not total reuse - the name and part of the personality will be different and they might morph to fit a different setting. But when you're DM in need, a stat block is a stat block.

Since I have been running games in Aquerra since 1989, this has happened quite a bit.  Players love to see their old PCs (or the PCs of other players they know) show up as NPCs.

Or having an NPC that was a villian in an old game show up as a possible ally in the current game, or vice-versa.

Yes, I do.  Some of the more memorable fights I have run were between one group of PC's run by the players and another group of PC's from a different campaign run as NPC's by me.  A good college gaming club was excellent for this.  Tactics used by your party used in one game could be turned on a different group of players later in the week when you were running things.

My very first D&D party often shows up in my Greyhawk games.  So much so that Darlena just expects to see Kromm, Vildhelm, and Zebulon somewhere.

I mostly cut that out when I realized that they were DMPCs.  Not by my design, exactly -- they were so ridiculously overpowered that the PCs would do anything they could to keep them around.  It was quite logical from their point of view; the powerful NPCs were walking tanks and/or air support, so it made the PC's lives much easier.  

So I'm not doing that any more.  I've been bouncing between settings and even game systems lately, so there hasn't been much chance to have recurring characters.  The closest I've got is I'm running Lando as an NPC in my Star Wars game.  He keeps a connection with the original trilogy *and* he's not an uberpowerful character.  If the party relied on him to fix things they would be in pretty deep shit pretty fast.  

I do it all the time.

All the time. In fact, several ofthe major players in a current campaign were player characters in a campaign played in the same world when we were teenagers. That said, I tend not to recycle "henchmen" characters, but rather have some fun by "going beyond the ending" of the previous campaign.

I do it all the time.  One of my favotites is still my longest running group from high school.  By this point the characters are pretty powerful.  So I put a lot of them into position of authority within the Forgotten Realms, sometimes replacing a canon chracater sometimes just putting them in a undetailed position.

I will even occasionally bring back old PC's that were killed in an early game as NPC's.  Its a great as the player slowly realizes that teh NPVC that their traveling with is a PC of theirs from a two or three years back.

When I can... In my last D&D campaign, a dwarven cleric sacrificed herself to bull rush a Fiendish Beholder proto-god into a dimensional rift. If we ever decide to play in that setting again, she will definately make an appearance.

I haven't DMed my current group long enough to really be able to fit it in, though.

I once ran a very epic high magic fantasy game with very Iconic characters.  Unfortunatly I tried using the d6 rules from Westend and make up my own magic system.  The lack of a solid ruleset killed the game but not before the players got to know thier characters fairly well.  
A year or two later Hatter was going to be running a fantasy game and we conspired to have the pantheon for the world be made up of the player characters from my game having replaced the dying old gods and becoming deified after years of adventuring.  Except one player who had come into possesion of an artifact while playing that kept him from dying as long as he wore it.  He was going to be a mysterious old man that would advise but not help the players.  We wanted to see how long it would take the group to figure it out.

Yep, the gm box comments....only thing that sucks is we lost one of the boxes, though it was the larp box....

Quote from: NightfangSpecifically, do PCs from previous campaigns find their way into your current game as NPCs?

whenever possible without getting ridiculous about it.    cameos are fun!