Well, it's anyone he ever met before M-Day. That just accounts for his old high school teachers from Chicago and anyone in the X-Men who was his teacher. So that's...
-The knowledge of people who went to college for a few years.
-Beast's medical and scientific knowledge
-Wolverine and Shadowcat's martial arts (he wouldn't have accessed Psylocke's because his powers were telepathy based, and he couldn't glean skills from any psychic who kept their mind guarded)
-Nightcrawler's fencing and theatrical skills
-Gambit's pickpocketing and card-throwing (which should come in handy if Prodigy ever finds a top hat to throw cards in)
-Shadowcat's hacking skills (assuming her slight telepathic resistance didn't stop him)
-Iceman's... uh... accounting skills and knowledge of tax codes.
-Wolverine and Archangel's notorious skills in the sack
Basically, he could defend himself in a hand-to-hand fight, fix stuff, and operate on the injured. I'd say Cyclops' tactical skills were more of his method of thinking and less of a learnable skill, so Prodigy was always relying on what his teachers assumed to be his own tactical mind. He's probably the last person you'd want in a fight against anyone with superpowers, so I could see plenty of situations where his absorbed skills couldn't be used as a plot device.