Gizmo is yet another VoIP client for Mac OS X, but using SIP (actually based on SIPphone's own efforts)

Despite all the current hype, the client is pretty damn useless, since it will only talk to Gizmo's own service. So much for their touting Open Source and standards compliance - it's impossible to establish whether or not it's truly standard unless it can connect to standard SIP servers.

Update: Beware their Spam mail. Despite not wanting to receive any updates, I still got stupid messages listing "ten tips to do with Gizmo" and other junk. For me, that's more than reason enough to avoid their service.