Parkmans and Petrellis, below the cut. Spoilers through early season 2. Some non-spoilery Firefly references.
Petrellis & Friends:
Peter Petrelli loves Firefly and considers himself a Browncoat. He watched every episode when it came out, and he's seen Serenity more times than he cares to count. He identifies with the Tams in particular. He tried to get Nathan into it, but he said he had no time for things like that.
After the explosion, when Nathan was living in Peter's apartment, Nathan finally got to watching the DVDs, but he hasn't yet told Peter. As of now, he enjoys it but doesn't consider himself a fan.
Nathan wanted to name his daughter Madeleine, but Meredith overruled him. If he'd had daughters with Heidi, he would have pushed just as strongly for the name.
Monty has an ability, but won't manifest for another couple of years. Simon doesn't, but is capable of passing on the genes for it. Simon will always kind of feel like a failure for it.
Parkmans: (Parkmen?)
Janice's baby really is genetically Matt's, and they both know it. Janice is absolutely terrified that her baby will have some kind of power and face the same kind of things Matt is, so she lied to him. He played along partly because he wanted out, but mostly because he wanted to protect them. Neither Mohinder nor Molly know this, and Matt's almost been able to fool himself into thinking the baby isn't his.
Matt has a cousin who mysteriously disappeared for two days about ten years ago. When the cousin returned, he couldn't remember anything from that time. Matt never made the connection until a while back, but hasn't had the time to track down the cousin to ask if he has weird marks on his neck too. (At the time, everyone had assumed he'd run away and was too ashamed to admit it.)
Matt's mother is a nurse. She's only recently retired because she loves her work so much. She's thinking of moving to NYC to be closer to her son, but she hasn't told Matt about this yet. (Of course, he hasn't yet told her about the mind-reading, so they're about even on that.)