Calispora.org is a free and open "pod" (server) on the Diaspora Social Network. Every user has a home pod that they use to access the network.
Calispora offers many features not found on the main pod (joindiaspora.com and its clones) and lacks some features that are offered there. But actually, they are far more alike than different.
Just to be very clear, if you are looking to join a pod offering the "mainstream" Diaspora experience, Calispora is probably not for you. But there is nothing stopping you from joining multiple pods. You have multiple emails addresses, don't you? So why not join Calispora today? If you want a mega-pod that doesn't require an invitation, try diasp.org.
The main downside to joining Calispora.org is that if the core Diaspora developers significantly change the networking language (we call it the federation protocol) and Calispora is unable to incorporate those changes, we will be cut off from the main pods. It is a distinct possibility, but may never happen.
Still reading? Fantastic!
You can find a more complete list of features, but here are the main differences (as of July 4, 2012):
That's only scratching the surface. Join calispora today and give it a test drive. There's no obligation and you can close your account at any time. And it's FREE.
For a really nice list of available Diaspora pods, see podupti.me. Welcome to D*!