Also known as WSO2 Stratos and it’s all open source. It comes in two forms. You can either set it up on top of a private cloud used in an enterprise or you could try out the hosted service at cloud.wso2.com.

Samisa has written a post giving some background about how Stratos came to life. Infoworld has some more info.

Usually people don’t open up all their source code that goes into a hosted version. But the source code for WSO2 Stratos is open source and available with an Apache license. You can grab a source tarball here or checkout the source code using subversion.