Jingle is the XMPP extension that allows for peer-to-peer client connections to transmit audio, video and other data.
Many XMPP clients now support voice and video using your microphone and webcam. A list of clients that support Jingle can be found on the Wikipedia page.
Server support is not required for Jingle, as the clients communicate directly and only use the XMPP channel for negotiating. As a result Jingle clients are fully compatible with Prosody.
Google also uses Jingle for voice and video support in their clients. Their Google Talk gadget and Gmail chat support the current version of Jingle.
Unfortunately due to an XMPP compliance issue it is not currently possible to make a call from Gmail to a client logged into a Prosody server. Google sends an invalid XMPP stanza that Prosody rejects (see issue 248). Google are working on a fix.