You collect these "rare items" throughout the game. You can't use them at all, but you can look through them in the items menu. They play key roles at certain parts in the game and you keep them so you can remember their part in the plot.

Name
Location
Pharmacy Coupons Wall Market
Cotton Dress Wall Market
Satin Dress Wall Market
Silk Dress Wall Market
Wig Wall Market
Blonde Wig Wall Market
Dyed Wig Wall Market
Disinfectant Wall Market
Deodorant Wall Market
Digestive Wall Market
Cologne Wall Market
Flower Cologne Wall Market
Sexy Cologne Wall Market
Grass Tiara Wall Market
Ruby Tiara Wall Market
Diamond Tiara Wall Market
Member's Card Wall Market
Keycard 60 Shinra Corporate Tower Level 59
Keycard 62 Shinra Corporate Tower Level 61
Keycard 65 Shinra Corporate Tower Level 62
Midgar Parts Shinra Corporate Tower Level 65
Keycard 66 Shinra Corporate Tower Level 65
Keycard 68 Shinra Corporate Tower Level 68
PHS Kalm Town (This is used to switch the members of your party)
Basement Key Shinra Mansion
Keystone Dio's Showroom in the Golden Saucer
Black Materia Temple of the Ancients
Glacier Map Icicle Inn
Snowboard Icicle Inn
Mythril Sleeping man in the Eastern Continent Cave
Huge Materia #1 North Corel afteer stopping the Coal Train
Huge Materia #2 Fort Condor
Huge Materia #3 Shinra No. 26
Huge Materia #4 Downed Submarine
Key to Ancients Beneath the Northern Continent. You need to use the submarine