I can attest to Skype doing this. A friend away moved away for graduate school and we would communicate using Skype, so I started just leaving the desktop application open. My computer is located in my bedroom, with a switch next to it. I woke up like 3am, see the lights FLASHING going all sorts of nuts on my switch, which was weird as I had nothing on my pc open at the time. I check net stat... i see a inbound and outbound connection, one to some SBC DSL user in Atlanta, another to a Comcast user somewhere else, forgot where, but some other state. I kill Skype. BAM, connections close, traffic resumes normal operation. Skype was using my computer as relay service, since I have active UNPN, and the other two client presumably had some sort of firewall blocking direct communication. To this day i tell *EVERYONE* who uses the Desktop app to close it as soon as they're done to prevent this as most home connections now have meters. (Charter's is 250gb/mo for 30mbit, which I hit 150gb+ some months when I was toying around with AOSP and downloading the entire repo a few times after screwing up a VM or something).