ZetaGrid is a grid computing solution that is based on a standard 3 tier J2EE architecture using a client server model. The name comes from the first project that used/developed it - zeros of the zeta function.
It claims "ZetaGrid is also the first ever specification of simple J2EE interfaces to distribute applications securely and reliably in a cross-platform grid infrastructure" (strange claim?), and communicates "via stateless sessions using HTTP" (EJB?).
A great deal of the project effort seems to have gone into being able to make it work in a real world scenario. The project has been careful to give its participating systems full control of client resource consumption. The system is client driven, so no unusual firewall rules required, and takes into account the connection speeds and network architectures to communicate 'work units' effectively. The server side's big plus is that it functions as a J2EE application on an application server with a database making it easy for sysadmins to get to grips with.