Abilities Peter has/might have so far:
Charles Deveaux's power (which I believe is the ability to exist in dreamtime, but this is just my theory)
Angela Petrelli's power (unknown)
Flight: from Nathan Petrelli
Precognition: from Isaac Mendez
Telepathy: from Matt Parkman
Persuasion: from Eden McCain
Invisibility: from Claude Rains
Phasing: from DL Hawkins
Super strength: from Niki Sanders
Electricity manipulation: from Elle
Telekinesis: from Sylar
Cellular regeneration: from Claire Bennet
Space/time manipulation: from Hiro Nakamura
Radioactivity: from Ted Sprague
Abilities he might have but haven't been confirmed in show yet:
Organic Radar (my own name for it

): from Molly Walker
Technopathy: from Micah Sanders
Healing: from Daniel Linderman (this is possible in two ways but by no means certain: A) his empathic mimicry ability works even on dead people, or

Linderman has been to the Petrelli household before. Obviously option B precludes that Peter was absorbing powers earlier than his ability to use them kicked in, so it's unlikely that he has this ability at all)
Eidetic memory: from Sylar, indirectly from Charlie Andrews
Super hearing: from Sylar, indirectly from Dale
Melting stuff power: from Sylar, indirectly from Zane Taylor
Cryokinesis: from Sylar, indirectly from an unknown source
Regeneration/immortality: from Adam Monroe
Alchemy: from Bob Bishop (this assumes that he was absorbing powers when around Bob. He managed to get Elle's, so this is a possibility. Of course, there were other times he was around Elle)
Memory manipulation: from The Haitian (A much-discussed possibility that could be argued either way, but it's still possible he has it even if he doesn't know this)
And of course it's entirely possible that random people Peter has been passing in the street since the show began had an ability and he would have absorbed those too.