Payload Extractor

Payload Extractor is a utility that opens Mac OS X .pkg files, the same files opened by the OS X Installer application.

But unlike the Installer application, Payload Extractor allows you to decide which file(s) you install and where you install them.
For example, you could use it if you needed to replace only one damaged file in an installation, or if you wanted to install another version of an application without installing over the previous version.