One of the most popular choices for video communication is the chat program Skype, but there are several in-browser alternatives such as Tinychat and AOL's AV service. These services all offer direct instant video chat between multiple people on demand.
Skype allows for a large quantity of people to group together using voice and video chat, if desired, all hosted from a single user's computer and streaming to every person involved in the call. Skype requires that each user have an account and be running the Skype program for their specific device.
Other services can be used without registration and even without running an extra application at all. Services such as Tinychat and AOL AV can be run directly in each of the users' browser of choice and, while a bit more technically limited compared to Skype, can allow the same level of video communication between multiple users.
There is another more specialized category of video chat services that aren't used by pre-established groups to communicate but rather facilitate meeting new people. Services such as Chatroulette and Omegle allow users to go into a chat with a random stranger who is also using the service. From there, they can have conversations about whatever they like and may even have topics presented to them so that they have a starting point.