Displaperture 1.0
This one is for those that have upgraded to Leopard, but long for an OS X of long ago (or a few weeks ago, as the case may be). One minor change in Leopard, as seen to the right, is that the top two corners of the main menu bar are no longer rounded. They go right to the edge of your screen, which is a welcome change if you ask me. That unused space always bothered me, but if you're longing for those rounded corners long no more. Displaperture 1.0 brings those rounded corners back, and let you control just how rounded they are (and which corners get the rounding treatment).
Displaperture requires Leopard, and is freeware.