SharpDevelop is an IDE that supports WiX projects in a way similar to Visual Studio.
Ultimately, WiX source files are schema-validated XML so any good XML editor with syntax highlighting and schema-based editing assistance would work. WiX tools can be run directly on the command-line but WiX projects are generally more convenient. WiX projects are Visual Studio projects that are MSBuild projects. As with WiX source files, MSBuild project files are XML. The WiX installer and/or zip distribution provides MSBuild targets and tasks that call the WiX tools during a build with MSBuild, Visual Studio or SharpDevelop.
When learning WiX, do keep in mind that it is a tool to build Windows Installer packages from an abstract declaration. Go to the Windows Installer documentation for conceptual information and WiX documentation for how to implement the concepts. You'll then see how much simpler WiX is. However, compared to other installer tools, WiX is not simple-but it doesn't limit you, either.