GameShark

Play CD-R discs:
Note: This feature requires a v3.0 GameShark cartridge. Use a spring or similar object to always keep the PlayStation's lid button depressed. Enter the "Use Enhancement CD" option in the GameShark menu while a normal PlayStation game is in the system. It will try to play your disc, but will stop since it is not an Enhancement CD. A message prompting to press X will appear. Switch the disc with the CD-R, with the spring keeping the lid button depressed, then press X and exit out to the main menu. Start the game and the PlayStation game will begin to load.
Information in this section was contributed by Snailian and Sean.

Note: Creating a copy of a PlayStation disc is considered illegal unless you also own the original game disc.

Message from Gav:
This trick requires the GameShark CDX 3.3. Hold L1 + L2 + R1 + R2 at the main screen. The directions that scroll at the bottom of the screen will stop and a message from Gav will appear.
Information in this section was contributed by Roy Kinney.

Message from Gavin Thornton:
This trick requires the GameShark version 1.1. Hold L1 + L2 + R1 + R2 at the main screen to display a hidden message from Gavin Thorton (the programmer) at bottom, in the scrolling message bar.
Information in this section was contributed by Roy Kinney.

Hint: Backup boot disc for CDX:
If you have both a GameShark CDX and a Sharklink (which comes with a disc), you can use either disc to boot the CDX memory card.
Information in this section was contributed by Chris Sanicola.