Enter any train station and find used metrocards littering the floors like discarded pennies. Which in a way they are. Most people don’t realize but lots of those thrown away metrocards have money on them.
In the past, I’ve found everything from .05 to $4.50 which alone means squat but bring them to any tollbooth agent and they’ll combine up to 4 metrocards for you. Combine enough cards with leftover change and you'll be riding the train for free!
I’ve done this dozen of times and I’ve rarely had any problems.
Before the MTA $1 new card surcharge tossed metrocards were easier to find. While people are reusing metrocards more you can still find some if you are vigilant. Check the floors of train stations, buses, or even the streets.
My advice: if you have more than 4 cards with money on them take the ones with the most money and combine those first then go combine the rest of your cards at another station! Don't bother with cards that are bent! Impossible to read and tollbooth agents won't take them!