If you're disappointed about the loss of the rounded window corners in the Leopard menu bar (and the loss of the rounded bottom corners in Mac OS X in general), get Many Tricks' freeware Displaperture.
Displaterture allows you to choose which of the four corners you want rounded, and also lets you pick the radius. It is headless and doesn't appear in the Dock.
[robg adds: You can also use this nifty little app to turn your machine into a great impersonation of a 1950s-era television set. Try it and you'll see what I mean.]

