Open source Twitter AIR application Spaz has added supported for Identi.ca and Ping.fm. The support though at this stage isn't for the non-technically inclined, with both requiring edits to Spaz's code to work.
To use Identi.ca via Spaz, users must manually edit the preferences.json file, swapping twitter-api-base-url and twitter-base-url out for identi.ca/api. The genius though of the change: forward compatibility to any future microblogging platform that implements the Twitter API via two quick changes in a file, as opposed to having to go through the entire app and swap out API references.
Ping.fm support also requires edits to the preferences.json file. Needed to work: add to the services-pingfm-userappkey and enable services-pingfm-enabled.
I haven't used Spaz for some time, and it's become quite impressive, helped by an active development team. The service is multi-tabbed and features most of the extras lacking in most Twitter clients, but standard in Twhirl. One particularly nice feature is the ability to custom tweak the CSS layout.
Until such time Spaz makes Ping.fm and Identi.ca support easy, thousands won't be flocking to use it, and Twhirl offers the easier choice of Identi.ca support. However, if you're not afraid of a bit of coding and love the idea of an open source microblogging application, give Spaz a shot.