WiX Toolkit from Microsoft Released 

Windows Installer XML (WiX) toolset has released as Open Source on SourceForge.net. This is the first Microsoft released product to be under an Open Source License. Bascially its a set of tools for creating MSI Installers using XML. There is a complier called Candle, a linker called Light, a lib tool called Lit, and a decompiler called Dark (cute huh?). Anyway, here is the link on Rob Menshing's Blog about it, and here is the project link on SourceForge. Looks very promising.