Ainethach - Breton, Karthwwasten
(Must do the quest "Convince Atar to leave Sauranach Mine" and side with him first).
Angrenor Once-Honored - Begger, Windhelm
(You must give him a gold piece first).
Athis - Dark Elf, Jorrvaskr, Whiterun
(Must complete "Glory of the Dead"; trains in One-Handed; if follower, you can train, then take the money back).
Argis the Bulwark - Vlindrel Hall, Markarth (Housecarl of Markarth, you must become Thane of Markarth first).
Balimund - Nord, Blacksmith, Riften
(Must bring him 10 Fire Salts for his forge first).
Belrand - Battle-Mage, The Winking Skeever, Solitude
(He is a mercenary, you must hire him first).
Benor - Nord, Morthal
(You must best him in a brawl first).
Calder - Nord, Housecarld of Windhelm
(Must join the Stormcloaks and complete "Rescue from Fort Neugrad" first, then complete 5 tasks for the people and purchase Hjerim).
Cosnach - Breton, Silver-Blood Inn, Markarth
(You must best him in a brawl first).
Erik the Slayer - Nord, Barbarian, Frostfruit Inn, Rorikstead
(You must convince his father to let him be an adventurer first). (Known Bug: choice for marriage may not come up. It did come up in my first game, but in later games it did not, very frustrating for me since I fell in love with him! I wrote on several forums and to Bethesda itself, and was VERY disappointed when Patch 1.5 failed to fix)
Farkas - Nord, Jorrvaskr, Whiterun
(Must complete "Glory of the Dead' first; trains in Heavy Armor; if follower, you can train, then take the money back).
Filnjar - Blacksmith, Shor's Stone Ghorbash the Iron Hand - Orc, Dushnikh Yal (You must Persuade/Bribe/Intimidate him to travel once more first).
Marcurio - Imperial, Battle-Mage, The Bee and Bard, Riften (He is a mercenary, you must hire him first).
Moth gro-Bagol - Orc, Understone Keep in Markarth
(You must bring him a Daedra heart first). Octieve San - Nord, Winking Skeever in Solitude (Must get Irnskar to drop his debts first).
Omluag - Breton, Miner in Markarth
(You must complete the Misc. quest "Talk to Mulush about Omluag" first).
Onmund - Nord, student, College of Winterhold
(You must first convince Enthir to give back Onmund's amulet.) (Note: To do this, you can Persuade and he'll just give it to you if your Speech is high enough, but if not, he'll send you on another quest to get a staff for him.)
Quintus Navale - Imperial, Alchemist, Windhelm
(You must complete "The White Phial" and "Repairing the Phial" quest first). (Possible Bug: May not be able to buy/sell once married - worked for me first time)
Revyn Sadri - Dark Elf, Merchant, Sadri's Used Wares, Windhelm
(You must sneak Viola's stolen ring back into her house, but DON'T snitch on him).
Roggi Knot-Beard - Nord, Miner, Kynesgrove (You must find his ancestral shield in Stony Creek Cave first; he will also be a follower once you return his shield).
Romlyn Dreth - Dark Elf, Meadery Worker, Black-Briar Meadery, Riften
(You must take a box of mead to someone in Solitude first).
Scouts-Many-Marshes - Argonian, Works at the Docks in Windhelm
(Must do a favor for him first).
Sondas Drenim - Dark Elf, Miner, Goldenrock Mine, Darkwater Crossing, Eastmarch
(You must deliver his medicine request to Quintus Navale first).
Sorex Vinius - Merchant, Winking Skeever, Solitude
(You must complete a delivery quest - taking some wine to the Blue Palace, first). (Known Bug: When I talk to him, wearing Mara's amulet or not, he never gives me his quest, to take some wine to the Blue Palace.)
Stenvar - The Candlehearth Hall, Windhelm (He is a mercenary, you must hire him first).
Torvar - Nord, Jorrvaskr, Whiterun
(Must complete "Glory of the Dead" first).
Vilkas - Nord, Jorrvaskr, Whiterun
(Must complete "Glory of the Dead"; trains in Two-Handed; if follower, you can train, then take the money back).
Vorstag - Nord, Mercenary in The Silver Blood Inn
(You must hire him first for 500 Gold).
Wilhelm - Nord, Innkeeper of Vilemyr Inn in Ivarstead (Must complete "Investigate Shroud Hearth Barrow" first).