NInstall
NInstall is an application to create and edit Microsoft Installer (MSI) files. The main project goal is to provide developers with an Open Source tool that can be used instead of the expensive commercial tools that are available.
The first version of the application will probably never be released but the code in SourceForge will allow you to create MSI files. The UI is limited to editing the tables that will be used to make the MSI file, and if newer versions are ever released the UI will improve to make editing data easier.
Development is currently stalled, but may pick up again in the future.
If you have any questions then read the project FAQ.
Links
Microsoft Installer Documentation