Of all the people who mention One Piece in the series, I was trying to think of the people who DON'T mention it, and therefore it reasons that they may know something about it:
1. Shanks: Not once has Shanks made a mention of One Piece OR Gol D. Roger. Or even Raftel, for that matter. As far as I can tell, One Piece is Shanks' hat. (That would be fantastic, if Luffy and crew went all the way to Raftel to find out that One Piece was the hat Shanks gave him at the beginning of the series, aka Roger's former hat) XD Seriously, though, I think Shanks knows something about One Piece that he's not letting on about.
2. Whitebeard: Ace does, but Whitebeard doesn't mention One Piece, finding it, or whatever. Do he and Shanks have an idea of what One Piece already is?
3. Arlong: Correct me if I'm wrong, but Arlong never mentions One Piece. Having lived on the Grand Line for many years, you think Jimbei knows a little more about One Piece than we think he would? Being a Fishman King and all, I'm pretty sure he could be the best suited to scouring the seas for a legendary treasure.
4. Gorosei: The Gorosei don't mention One Piece, though I do think they mention Roger from time to time.
5. Mihawk: I think Mihawk speaks to Luffy about being KAizoku-ou, but not directly about One piece. Maybe Mihawk knows something about it, too.