MBONE: Multicasting Tomorrow's Internet
There are two answers to that question: nothing, and a few hundred dollars. It really depends on the degree of participation that you desire.
To receive MBONE events, an ordinary UNIX workstation, a PC running Windows, or a Mac will do. This is the "nothing" part of the answer. No special hardware is required to receive MBONE sessions.
To send audio, a microphone is required, along with sound hardware. Sound hardware is already included with most UNIX workstations and Macs. It's possible to buy very cheap sound hardware for the PC, and one brand that comes to mind is the de facto industry standard Sound Blaster card.
Sending video data becomes a little more expensive. A frame grabber card and a camera are required. For example, on Sun workstations, the VideoPix card is already used to send video to the MBONE. Other frame grabber brands include Parallax and J300, to name a couple.