I've just come back from Marrickville organic food markets and managed to pick myself up a nice little cactus for free of one of the vendors, I'm not sure what it is and the guy who gave it to me had no idea. I've done a little research and think it could belong the Mammillaria genus, anyone care to have a guess?

Hello there...
That's Mammillaria gracilis. It will grow into a mound and pieces will fall off and root down easily...Has small white-cream flowers, but can be a little reluctant to bloom...