Fullscreenizer is a small tool to force a game into a borderless windowed fullscreen mode (remove the border from a game's window and resize it to cover the entire monitor).
Strongly inspired by the app of the same name by Kostas "Bad Sector" Michalopoulos. You can find the original program here.
Using fullscreen mode in some games can cause issues with multiple monitors or when alt-tabbing, some old games don't support the full resolution of modern monitors, and a plethora of other issues. Borderless windowed fullscreen mode is a good alternative that solves most of these problems, but not all games support it.