Orca is available for free from Microsoft. Orca allows to either edit a MSI file directly, or to store modifications as MST files. See Orca for information on how to get it from Microsoft.

InstEd is a Free msi editor built for professionals.

SuperOrca from “Pantaray Research Ltd.” is a direct replacement to the “Orca” MSI Editor from Microsoft. SuperOrca may be used to examine and modify an MSI database in order to distribute a new MSI package.