A great headphone splitter is on sale on Amazon today for $10.50.
I used headphone splitters a lot when traveling with friends. before US Airways got internet on most flights, and I still use them internationally for movies I have queued up.
A note about splitters though–be careful if someone unplugs because your volume will up tremendously.
This one also lets you plug multiple audio sources in–just in case you are into multitasking or creating mash-ups in the moment.
Disclaimer: Since this is one of Amazon’s special deals, I don’t believe I get affiliate credit,