Mumble isn't a Skype alternative. In contrary, Mumble is a TeamSpeak/Ventrilo alternative. Mumble is only useful if you really have an appointment/meeting to attend in some server or if the video game server you're playing provides a Mumble server to connect to, or any variation of these.
This is due to the fact that with Mumble (and others mentioned here), you **must** be in the same server as the person you want to talk to, and at the same time (no offline messages). Besides, this brings about the whole question of how is the communication made. That is: Even with Jitsi (unless you both use XMPP/Jabber accounts in it or some other client of choise other than Jitsi, as long as both accounts are XMPP/Jabber), you'll probably have problems when trying to communicate in cases where both of you are in different servers, or in cases where the service provider is having difficulties providing the service. Jitsi (the client) isn't a service provider by itself, but a client for various protocols (one of these being XMPP/Jabber). As for Jit.si, as far as I can tell, it seems to be a network service that might be using a technology similar to WebRTC, however, it seems to force non-free software (due to non-free JavaScript).
