Apple 661-0121 Floppy Disk Drive. The second and newer model of Apple's SuperDrive. This 3.5" floppy disk drive has a black door and is "manual inject" (or manual insert) and used in all PowerPC Macs. Late 68K machines may also use this drive, but the cover, case or bezel will differ between "auto inject" and "manual inject", so they are not interchangeable.
Pictured and most common is a Sony 2MB, Model MPF 42A, but another drive sharing Apple's same part number and completely interchangeable is Mitsubishi MF355F. You may receive either, note that these likely do not display any Apple identification on the drive.
Drives tested to format, write, read and eject easily.