There's nothing quite like inviting candidates to come in and meet the people they will be working with in a semi-informal session. Participants log into a specially created site to view slides and participate in a simultaneous telephone conference call. Additional interaction is available via online limited chats and forums. The amount of interaction with attendees is largely confined to telephone-based interaction and the number of people you can effectively manage on one conference call. Pluses include fast set-up, low fees and relatively easy access.
Most models require registration of some kind.
Typically these events allow for a more polished production and make it easier to accommodate larger numbers of audience members - especially if you intend to make a recorded version of the show available at a later date. Cons include a higher price point and more work in planning and hosting the event. The technical obstacles to hosting your own streaming media presentation are also considerable. In other words, unless you're a Cisco or PWC, don't even think of hosting your own event, as unexpected numbers of viewers can easily overwhelm an ill-prepared server farm. As with most Web applications, using technology to supplement a real world experience is often the best bet.