I've had several, but I've never really had them seek the PCs out, it's usually been the PCs seek them out and then they point the PCs to something. I certainly don't have some Elminster-type randomly show up to save the PCs from certain doom and metaphorically allow the DM to stroke his ego's giant wang. But here's a number of NPCs that the PCs have used as directive forces at points:
Tollysalmon - a githyanki and insane former factol of the Bleak Cabal in Sigil, currently incarcerated in the Irretrievably and Criminally Insane Ward of the Gatehouse. She happened to be there incidentally and said something in passing to the PCs in my last campaign, something which seemed to indicate that she somehow knew a lot about what was then going on, despite having absolutely no connection to the events (it was a bit of random creepyness for atmosphere). Well, this second campaign now, the PCs have gone back to her a couple times and as it turns out, she's rather involved in the plot herself in some twisted little way.
Rhys - former Factol of the Transcendant Order, current chairwoman of the Sigil Advisory Council. The PCs asked her advice on a number of times, and each time they came out ahead. She never once led them astray when they actually needed the direction and sought her out, but it always seemed like she'd anticated that they'd be there looking for her.
The Cheshire Fiend - a yugoloth who interacted with the PCs by virtue of a semi-illusory projected avatar that looked like a large glowing fanged grin. For a few years of the game, they weren't honestly sure if they trusted him or if they hated him. He never harmed them, but they got used to his benefit enough, and mislead just as much as given useful information, that he was probably more trouble than he was worth. Plus, he was the child of the Keeper of the Tower Arcane back in Gehenna.
Daru Ib Shamiq 'The Lie Weaver' - yeah, it's probably not a good idea to seek out and ask advice from and trust a guy named 'The Lie Weaver'. But they didn't have much of an option given the sort of information that they wanted, and after one absolutely lovely wild goosechase he sent them on, they later went and sought out his brother 'The Blind Clockmaker'. That turned out swell as well...
Chorazin ibn Shartalan, the Thrice Damned - third time's the charm perhaps. This particular fiend didn't screw them over, and his help quite literally made some good happen. He didn't screw them over because they bailed on him before he had the chance.
However... more commonly the veritable mouthpiece of the DM has been something small and cute/annoying. Fairy Dragon, Pseudodragon, Quasit, Imp, Nalg, Mephit... pick one of them. That seems to be a schtick of mine, having something along those lines as my own little in character peanut gallery in my campaigns. I suppose Nisha the tiefling xaositect might qualify in the same way.