Displaperture brings back rounded screen corners to Mac OS X
Displaperture brings back rounded screen corners to Mac OS X 10.5 "Leopard" and newer. Choose the corners you want rendered as round, choose their radii, and enjoy CRT nostalgia. You can round the corners on all attached displays, or just the one with the menu bar.
Details
Displaperture 1.3.2 runs natively on Intel-based Macs and requires Mac OS X 10.6 or newer. Users on 10.5 or earlier, or on PowerPC Macs, will want to use Displaperture 1.1.
http://manytricks.com/displaperture/