Super Mario Bros. X (“SMBX”) is a fangame based on several Super Mario Bros. entries. It was created by Redigit, who would go on to develop Terraria.

Ever since the game was litigated to nonexistence, new forks have sprung up:

• SMBX2 is an evolution of SMBX with Lua scripting support. It probably supports SMBX level packs.
• TheXTech is a cross-platform port of the SMBX engine, which also distributes SMBX itself.

• History of SMBX on Moondust Wiki