All of my cats purr, but some are more reluctant purrers than others. Galileo purrs when he's sitting on your lap, or lying with you in bed, or when he flops over for tummy rubs.
I don't hear Dante purr a lot since he's a bit standoffish; my parents are his "special people". He does purr when he's stomping on, and getting a bit too frisky with, something soft, or on the rare occasions when he's feeling cuddly, though. Dante purrs when he wakes my parents up in the morning (he sleeps on their bed), and when he sits with either of them, usually on the arm of the chair they're occupying.
Cosette only really purrs when she cuddles with me, and I'm really the only person she cuddles with. My bedroom was Cosette's isolation room when she first came to live with us as a kitten, so she bonded to me.
Autumn purrs when she's rubbing up against people's legs, which she does pretty much all the time when she's not off sleeping somewhere. Autumn has no desire to be picked up, and she only rarely deigns to sit on my mother's or my lap, but she's a very affectionate cat, and she follows me or my mom around all day, rubbing against our legs and being generally demanding of our affection. Autumn would be a terrible cat for an older person--she's such a tripping hazard. :roll:
Ramona is less demonstratively affectionate, but she very rarely will come sit on my legs if I'm watching tv on the downstairs couch, and she purrs then, especially if I scratch her chin. Ramona will also purr and rub legs when she wants something in the immediate vicinity--usually cat milk when we're in the kitchen and to play when the Cat Catcher toy is nearby.
Choco-cat purrs when she's near me.

She's especially purr-y when I first wake up and pet her on the bed before we get up.