Gizmo Project is not opensource, but open standards (contrary to some shoddy media reporting) and therefore may interoperate farily well (not perfect) with Asterisk using SIP. Details on how to configure for use with Asterisk are available here.
For those of you who do lots of US calling, Gizmo has announced US$0.01/min for calls to the US. And now, I noticed this on the Michael Robertson minute:
Gizmo is not perfect, but if you are looking for an IM/softphone to use in conjunction with Asterisk it is worth exploring, especially with the recent announcements. One of the biggest items missing is the ability to connect to other SIP proxies (ie - Asterisk) and register directly, maybe one day…
Also, it is possible to patch your Asterisk box with AstJab and use your Gizmo IM status to make routing decisions in your dialplan. I have used AstJab against Jive Messenger only, but based on the fact that Gizmo uses an open XMPP server for presence/IM it should work without a problem.