Bartolo Ginsetti was an arrogant clothier in Old Hillsbrad who wished to "be remembered." Today, he gives Alliance a quest where he makes them a yeti-fur cloak.
Kel'Thuzad was a Kirin Tor member who discussed the finer points of necromancy with Helcular while strolling through Southshore. Long story short, he delved into dark magic, was killed and resurrected by Arthas, and has become an extremely powerful lich. In WoW, he's Arthas's top minion and the end boss of the Naxxramas instance.
Helcular was a mage who wanted to learn how to become a necromancer. Apparently he succeeded, as he was killed by a lynch mob while trying to turn himself into a lich. Hordies get a quest to resurrect him so he can have his revenge on Southshore.
Herod the Bully was a tough-looking but lonely kid. Nowadays he's a leader of the Scarlet Crusade and waits for adventurers in the Scarlet Monastery's armory. BLADES OF LIGHT!
Sally Whitemane led a group of three children in playing interrogation games. She grew up into a very attractive young goth lady and took the position of High Inquisitor in the Cathedral of Scarlet Monastery.
Renault Mograine was Sally's loyal playmate and minion in childhood. Today he fights alongside her in the Scarlet Monastery, re-enacting their childhood games. He also killed his dad (more on that later) and was later killed by his dad's ... ghost? Spirit? Whatever.
Little Jimmy Vishas tagged along with Sally and Renault as a kid. He also hangs out with them as an adult, as the lead interrogator for the Scarlet Crusade. Naughty secrets, anyone?
Nathanos Marris was a human ranger lord and Alliance tactician, until the Scourge killed him. Today he "lives" at an old farm in the Eastern Plaguelands and serves as the Champion of the Forsaken. Horde will quest for him a lot -- Alliance will try to kill him. You know, it's amazing how many people haven't changed their clothes in seven years, although I guess the undead have an excuse.
Raleigh the True hung out with Nathanos and Bilger in the Southshore Town Hall. He left the Scarlet Crusade and now gives Alliance a quest to kill its leaders. He's a good reminder that male pattern baldness makes fools of us all.
Bilger the Straight-Laced was apparently too uptight for his own good. He's now Brewmeister Bilger in present-day Southshore.
Stalvan Mistmantle was sleeping soundly upstairs in the Southshore Inn. He wouldn't sleep soundly for the rest of his life, as he fell in love with one of his students, killed her and a bunch of people with an axe, and became an undead in Duskwood. You get what is seriously one of the longest quest chains in the game to kill him if you're an Alliance.
Zixil was building Overwatch Mark 0 down by the docks. They now roam Hillsbrad Foothills, selling lowbie recipes to Horde and Alliance alike.
Magistrate Henry Maleb has great job security, since he's been the leader of Southshore for 7 years now.
Nat Pagle used to fish down by the docks, where oddly enough, there are no actual fish. He passed the time by predicting strange events, such as the fall of Archimonde, the undead attack on Tarren Mill, Arthas's betrayal, and Thrall's escape from Durnholde. In a little bit of fourth-wall breaking action, he also knows the developers. He's now the most renowned fisherman in the world, and you can find him in Dustwallow Marsh.
Hal McAllister never left his fish business in Southshore.
Captain Henry Sanders talks to Hanes about his "hidden treasure" of a sash, a shirt and an eight-slot bag. You can find his treasure today by completing a quest that starts with a drop off murlocs in Westfall. He is missing, presumed dead.
Captain Edward Hanes helped Sanders hide his treasure. He is now an undead who gives Draenei quests on Bloodmyst Isle.
Commander Mograine, the founder of the Scarlet Crusade, forged the Ashbringer from an evil orc artifact. Unfortunately, he was killed by his son in an ambush and is now a minion of Kel'Thuzad -- specifically, one of the Four Horsemen in Naxxramas. If you get the Corrupted Ashbringer in Naxxramas, you can bring it to Scarlet Monastery and watch his spirit kill his son. However, you also get attacked by the Argent Dawn and have the sword constantly whispering to you, so that may be a negative thing.
Tirion Fordring was a paladin of the Silver Hand who saw the making of Ashbringer. Later in life, he was exiled for saving an orc, Eitrigg, in a reaaaaallly long story that is available in e-book form. He now lives in a shack in Eastern Plaguelands, where he'll give you several quests dealing with his son. He's currently trying to reform the Order of the Silver Hand to provide a non-crazy-Scarlet-Crusadish force against the Scourge.
Taelan Fordring was just a kid playing with his father's hammer in Southshore. He joined the Scarlet Crusade with noble intentions, but was killed by Isillien when he tried to leave in the "In Dreams" questline.
Fairbanks was Mograine's most trusted advisor. After seeing his mentor killed by his son, he tried to tell the Scarlet Crusade about it and got murdered and hidden in Scarlet Monastery. You'll fight his undead corpse in a secret room behind Whitemane and the younger Mograine.
Arcanist Doan, another Scarlet Crusade member, can also be found in Scarlet Monastery today. He's the last boss of the Library. I am ashamed to admit that I once took the Hypnotic Blade from him. I was a young rogue! I had no idea that Intellect was useless! Forgive me, Arcanist!
Isillien watched the Ashbringer's making. He later joined the Scarlet Crusade as their Grand Inquisitor and killed Taelan Fordring. However, Tirion Fordring killed him in revenge.
Abbendis also joined the Crusade, and keeps watch over the Plaguelands from Tyr's Hand.
Phin Odelic was a Kirin Tor mage studying in Southshore. He left the Kirin Tor, but stayed in Southshore as an Alliance questgiver.
Finally, Young Blanchy from the Tarren Mill stables is now Old Blanchy in Westfall. C'mon, horses don't live THAT long.
Credits to Wowhead and WoWWiki for some help with the "current" WoW images, since it's hard for a Hordie to get into, say, modern Southshore. Also, thanks to the KYL Advisory Crew (shown above in the instance) for first taking me to Southshore, and to Friggas Ring for reminding me of it and suggesting an Easter egg hunt!