XAP files show up in compiled Silverlight applications. Similar to the way a JAR file is like a TAR file, a XAP file is really just a ZIP file. You can rename it to ZIP, unzip it, and then feel free to decompile the DLLs inside using a suitable tool such as Lutz Roeder's .NET Reflector from redgate.

See: reverse engineering of Silverlight XAP files - Hannes Preishuber