I did a little comparison between Twitter and identi.ca, two micro-blogging sites available today. The list is pretty random and may contain errors, or may miss some items. And both sites are introducing and ditching features which could invalidate the list below. But feel free to let me know if something needs to be changed (I'm @bram85 on identi.ca and Twitter), or use the comment section below.

	Twitter	identi.ca
Open source
Groups
Tags
OpenID
Jabber (XMPP)*
Notices to SMS**
Autosubscribe followers
@Replies filtering
Update protection
Theming
Mobile interface
Non-english interface***
RSS feeds
ATOM feeds
FOAF feeds

* Regarding Jabber support, Twitter used to have this, but silently ditched it.
** Differs from country to country. It appears that the range of Twitter is more limited than identi.ca's.
*** Twitter offers only one additional language besides English, Japanese. Identi.ca offers a wide range of languages.