Identi.ca runs on open-source software called Laconi.ca. It supports the Open Microblogging Standard, and in my opinion, is better than twitter. There’s 2 main reasons I say this:

1. It doesn’t have the same downtime as twitter. A microblogging service that has downtime as often as twitter doesn’t present
   a good service. If I’m out, and I want to update my friends, I expect to be able to fire up LaTwit (my application of choice for
   my phone, as it supports Laconi.ca AND twitter).
2. It isn’t limited to one site like twitter. Identi.ca runs on laconi.ca as does The TWiT Army & Open Microblogger,the beauty of it however, lies in the
   ability to subscribe to users on other laconi.ca based sites. So lets say that army.twit.tv/randomguy123 wanted to subscribe
   to my feed at identi.ca/joeb454, that’s totally possible. Then I can subscribe to randomguy123 over at TWiT Army and receive
   his updates on identi.ca.

Number 2 is what I like the most, if/when laconi.ca based microblogging systems catch on, or other microblogging software begins to implement the Open Microblogging Standard it should allow for a huge network between users, no matter what service they choose to sign up to. Although the networks can be closed as well from what I’ve read. You can stop people signing up to your laconi.ca installation if necessary, and of course if you want to restrict who follows your notices, simply block them, and then you have a semi-private network.

Personally I’d like to see more people move over to laconi.ca based systems, and status.net (a recently announced project) looks very promising. I’m also pretty interested in the development of laconi.ca, as it already has some awesome features (the XMPP service for one), but time will tell, so I’ll sit tight and see what the future holds for Open Microblogging