I have a very small collection I've picked up over the past 10 years of visits. Usually didn't have time to use the FP or found it on the ground. Sometimes I kept an invalid one just because they still had the ride logo on the top. I was in the MK on Jan 9 and since it was a low-crowd evening and near park closing, I pulled one for Buzz and one for SM just to keep, knowing that they were soon going away forever.
(BTW, I couldn't believe folks were waiting in a long standby line for Buzz when the machines mere steps away were giving FPs with return times in something like 10-20 minutes. Pull a pass, go on TTA or SM or Laugh Floor, walk right onto the conveyor belt in Buzz. I did that about 4 times in 90 minutes. Really showed me how few guests understood how to use FP.)
Something to add to a collection would be the special Fast Passes that were given out in that Year of Wishes or whatever it was called, maybe in 2007? If you got really lucky, you received a whole sheet of plastic FPs with open return times for that day, one for each of the rides in that park that used FP. At other times random CMs could hand you one of the cards with open times for a specific ride. My husband and I got one pass each for Test Track on that visit, and we saw one family get an entire sheet for each member of their party.