[Freeware] This authoring tool can create Windows Installer Packages.
[Description from the Vendor]

The Windows Installer XML (WiX) is a toolset that builds Windows Installation packages from XML source code. The toolset supports a command line environment that developers may integrate into their build processes to build MSI and MSM setup packages.

WiX is a free open source tool. Microsoft is the vendor of the tool and Rob Mensching maintains the WiX OpenSource Project.

Resources
Windows Installer XML (WiX) Toolset
Using the WiX Toolset to Integrate Setup into Your Development Process
WiX Tutorial - Introduction to the Windows Installer XML Toolset