As I've noted before, I'm a huge fan of Quick Look and I eagerly follow the third-party plugins released by developers. A new one came along recently that's worth a look, especially for the security-conscious out there. Suspicious Package will let you use Quick Look to examine the contents of standard installer packages before you install them. You can navigate folder structure and have a look at what it contains with one click.
Of course you can do this manually by right-clicking in the Finder and choosing "Show Package Contents," but this makes it that much easier to do a quick check. Unfortunately, it does not work yet on 'mpkg' meta-packages. Suspicious Package is a free download from Mother Ruin Software.
Update: As a commenter below notes, "Show Package Contents" only shows the contents, not where they will install.
[via QuickLook Plugins List]