PTLens

I've been using the PTLens Distortion Correction Plugin.  Its fast and easy to use.  It automatically detects and fixes lens distortion, straightening horizons etc.  It will also correct tall buildings or other vertical shots from "leaning" back.

Before...

After...

Check out the horizon line.

Plugins

In addition to the Nik Software Collection, I've been using Jade and Amber, two plugins for Aperture made by Datamind.  They are excellent.

Digital Film Tools' Ozone recreates the Ansel Adams zone technique.