You can hold ⌥ (Option) to make windows resize symmetrically, from the center. It even works when resizing from any corner in Lion. Update: You can also hold ⇧ (Shift) to resize a window proportionally.

As far as I can tell this is brand new in Lion, the first version of OS X that allowed resizing windows from any side.

Thanks to Kelly and Sören Nils Kuklau for help with this post.