Aela the Huntress - Nord, Jorrvaskr, Whiterun
(Must complete "Glory of the Dead"; trains in Archery; if follower you can train, then take the money back).
Borgakh the Steel Heart - Orc, Mor Khazgur
(You must Persuade/Pay off her dowry first).
Brelyna Meryon - Dark Elf, College of Winterhold
(Must complete the quest "Brelyna's Practice" first).
Camilla Valerius - Breton, Riverwood Traders
(You must complete the sidequest "The Golden Claw" first).
Iona - Honeyside, Riften
(Housecarl of Riften, you must become Thane of Riften first).
Jennasa - Dark Elf, The Drunken Huntsman in Whiterun
(She is a mercenary, you must hire her first).
Jordis the Sword-Maiden - Proudspire Manor, Solitude
(Housecarl of Solitude, you must become Thane of Solitude first).
Lydia - Breezehome, Whiterun
(Housecarl of Whiterun, you must become Thane of Whiterun first).
Mjoll the Lioness - Nord Warrior, Riften
(Must be level 14+ and complete the quest "Locate Grimsever" first).
Muiri - Breton Alchemist in the Hag's Cure (Only after you complete the Dark Brotherhood quest "Mourning Never Comes" and kill both targets).
Njada Stone-Arm - Jorrvaskr, Whiterun
(Must complete "Glory of the Dead"; trains in Block; if follower, you can train, then take the money back).
Ria - Jorrvaskr, Whiterun
(Must complete "Glory of the Dead" first).
Sylgja - Miner, Shor's Stone
(Must deliver her satchel to her parents in Darkwater Crossing, then return a satchel to her first). Temba Wide-Arm - Logger, Ivarstead (Must bring her 10 Bear Pelts first).
Uthgerd the Unbroken - Nord, The Bannered Mare in Whiterun
(Must beat her in a brawl first).
Ysolda - Merchant, Whiterun
(Must bring her a Mammoth Tusk first; you can find some in Halted Stream Camp and several other random bandit camps, so you don't have to worry about killing Mammoths if you're low-level. You could also be lazy and buy one from Belathor if he has one in stock.)