IBM Websphere Liberty
IBM Websphere Liberty is a widely-used modern lightweight server (less than 70Mb, I have several running on my PC) that allows modern Java EE development with Java 8. The server is free for development and build server, with limited production use (up to 2Gb of JVM heap space across all instances for the organisation). Web applications are deployed as standard web archives, which means it's easy to download and install demos of various frameworks and easy to use Maven. This is something I've fought with when developing OSGi plugins, because Maven is designed to manage and install dependencies be compilation. On the other hand OSGi best practice is to pull them from other plugins, defined in the MANIFEST.MF. At the very least OSGi needs the plugins adding to the classpath in the MANIFEST.MF. Tycho is designed to bridge the gap between what Maven expects and what OSGi expects, but it's not great. So a standard web application, sucking any jar files in via Maven, makes life a lot easier.