Cardfile is a personal information manager, with an interface metaphor based on the concepts of index cards, card files, and rolodexes, that was distributed with Microsoft Windows starting from the original version 1.01 until Windows NT 4.0 Server.

Cardfile was first released with Windows 1.0 as an application that would allow users to create and flip through index cards containing several lines of free-form text.

[1] Cardfile was also released as a 32-bit accessory for the early versions of Windows NT, using a modification of the Windows 3.1 file format but changing the signature to DKO and the character width for text data to 16 bits in order to support Unicode.

Third party applications exist which support opening or modification of Cardfile files for Linux[3] or even online.

[1] At the time of NT 3.51 development in the late 1980s, Unicode was fixed-format 2 bytes, now known as UCS-2 and considered obsolete as the later 1996 implementation of UTF-16 allowed for variable-length formatting.