Tetris Zero is a 2 player hack for the NES version of Tetris. In order to accomplish this, the ROM was converted from MMC1 to MMC3. The PRG was expanded to have 2 additional PRG banks.

Also, a custom bankswap routine was added since no bankswap was present in the original ROM. The following features were added:

Enabled 2 player mode.
Created custom controls for 2nd player on non & in-game screens.
Created custom designs for A/B-Type 2 player mode.
Created asm to allow 2nd player to have stats shown on in-game screen.
Player 2 has the ability to write their name into the scoreboard.
Added a new gameplay feature to the B-Type Mode. In B-Type mode, in order to win 25 lines must be cleared. There is now the ability to choose how many lines to clear. There is a choice from 01-80 lines to clear.
Player 2 now has the ability to obtain their own game piece, no longer sharing/stealing from player 1.
Player 2 now has a proper NEXT piece box, along with working select button to enable/disable display.
Player 2 now has the ability to select an additional +10 levels from what they choose from A/B-Type (player 1 can hold the A button down when they have level 0-9 highlighted, and when they press start with A button held, the player will start +10 levels from what was selected. Player 2 can also do the same thing. However, this can only be done while player 2 holds down the B button. When player 1 presses start, player 2 needs to have the B button held down in order to get the +10 level).
The archive also includes a .sav file for those who own a powerpak for the NES.