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Everything Must Be Paid for Twice
(www.raptitude.com)
Aside from torrent the other p2p where an open port can be important is Soulseek, you could be blacklisted by users from downloading pretty quickly if you don't/can't share music.
I switched to mullvad a few months back and what I noticed coming from AirVPN is how much less captcha requests I get, I'm not much into p2p so I'll probably stay with them.
It's nothing exclusive to open source. Eve Online removing boot.ini and bricking Windows installs was hilarious.