Displaperture rounds your display corners, which was a CRT nostalgia thing when we first created the app all the way back in 2007. Amusingly, though, it now seems modern again, thanks to the advent of rounded iPhone displays and newer macOS versions' fondness for rounded rectangles.

Displaperture 2.3 requires macOS 10.10 Yosemite or newer. The release notes are an interesting read.