I'm a terrible packrat - I save everything from my trips.
Ten years ago, my wife and I were in Paris. We'll be going back for a few days in October at the end of a trip to Spain and Portugal.
I have a packet of unused Metro tickets from our last trip. They're fairly simple little tickets printed on blue-green card stock, about 1" x 2.5" with a narrow magnetic-strip on the back. On the front, there's a RATP logo, with "Carnet 2 CL", "section urbaine" and 4 logos for M, Bus, RER and T printed in (now fading) black ink.
Are these still usuable, or should I leave them at home in my souvenir box along with the Francs and Escudos and Pisetas?