This key will unlock the door leading into the Chompers Dental Office in the southwest section of the Medical Pavilion. You're going to need to use your newly acquired Telekinetic powers to get it, though.
Obtained: Medical Pavilion - Obtained by focusing the Telekinesis plasmid at the Chomper Dental Office's "key return."
It seems as though the citizens of Rapture chronicled their entire lives on these audio devices. Most of the recorded messages will indirectly fill you in on the events that took place before Rapture's fall. Pay close attention - there's a lot of important information to be gleaned from the many diaries (122 in all!) scattered throughout the city.
Emergency Access Key
Dr. Steinman's key will unlock the Emergency Access gate at the end of the Medical Pavilion level, granting you access to another bathysphere.
Obtained: Medical Pavilion - Found on Dr. Steinman's corpse.
Julie Langford discovered a means to restore Rapture's plantlife before her untimely demise - the Lazarus Vector. You'll need to obtain 21 invention components before you can construct it, though.
Obtained: Farmer's Market - Invented at a U-Invent vending machine.
Without this key, you won't be able to pass through the gates leading from Arcadia to the Farmer's Market.
Obtained: Arcadia - Found in Julie Langford's safe.
R-34 Wire Cluster
In addition to two quarter-cans of Ionic Gel and a Nitroglycerin Charge, you'll need four of these clusters to complete the EMP bomb that Kyburz never had a chance to finish.
Obtained: Hephaestus - Found on Big Daddy corpses once you've discovered the EMP bomb.
Sander Cohen's Muse Key
If you kill Sander Cohen and take his key, you can use it to unlock Sander's Muse Box in the Atrium area of Fort Frolic. Inside, you'll discover 100 Dollars, 8 Incendiary Bolts, and 3 Spider Splicer Organs.
Obtained: Fort Frolic or Olympus Heights - Found on Sander Cohen's corpse.
Suit Control System
This is the first of several components you'll need to become a Big Daddy in order to resume your pursuit of Frank Fontaine.
Obtained: Point Prometheus - Found on Big Daddy corpse.