Terracotta recently acquired Ehcache and has released a tight integration of the Ehcache API with the Terracotta clustered store in a simple package and only requires a few extra lines of Ehcache configuration to go from single node to clustered, although you also have to run the Terracotta server process.
Ehcache with the Terracotta ES edition is open source and free to use. Commercial licenses are available if you want support, more scaling, indemnification, patch support, etc.
Terracotta does use a central server array, not a single central server, so there is no single point of failure! You can set up as many hot backup servers as you want and these backup servers can be configured to take over when the active server goes down. With Terracotta FX (commercial product), you can also use multiple active servers.
