Is it possible to use normal headphones for passengers in a light aircraft? I'm just finishing up my private license and am looking forward to taking some friends up in a Cessna 172.  I've invested in a nice aviation headset for myself, but, as none of my friends are pilots, they don't have headsets of their own.  Even the cheapest aviation headsets (at least that I've found) are around $100; friends going up for an occasional flight don't want to spend that kind of money, and I certainly don't want to shell out for several spare headsets.

My question is, are there adapters available so that normal headphones could be plugged into a standard 2 jack connector?  I have not been able to find any online.  While certainly not too effective at reducing noise, this would at least give my passengers some comfort and would allow me to communicate with them without shouting during these short and infrequent flights.  Worst case scenario, I could give them all earplugs, but it would definitely be helpful to have some kind of headset solution so that I'm not shouting through the sound of the prop and the earplugs and they're not shouting through my ANR.
There are some solutions to convert a normal set of headphones into an aircraft headset, but in total they are going to cost you more than a base level headset would.  

One popular (available in TSO and non-TSO options) option is UFlyMike which is an adapter for Bose noise cancelling headphones that adds a microphone and connects to a standard 2 jack airplane audio panel.  But back to your issue at hand, this is not cheap.  The UFlyMike will set you back around \$200 and the Bose headphones are another \$300, making this adapted headset cost a total of \$500.  This is a bargain compared to an actual Bose aviation headset, which runs around \$1000, but is well above an entry level non-ANR headset.

Your best bet is to ebay some old clamp style headsets (e.g. DC) to get some cheap headsets and then keep around your current headset when you eventually upgrade (unless those trade-up deals from e.g. Lightspeed are too hard to resist) to build up a stock.  Alternatively, if you are renting, you may be able to rent a few headsets with the plane so everyone can be equipped for a trip.