Real world uses of IP multicast

• Hoot 'n' Holler
• Historical background on the name and analog implementation that predate widespread computer networking.  See also links at bottom of that page, a couple of which are reproduced below. (Google: hoot n holler telecommunications)
  
• Cisco Hoot and Holler over IP - public Cisco documentation (Google: Cisco Hoot and Holler over IP Feature Overview)
• Distribution of market trading data
• Cisco documentation for those installing and operating a multicast-capable network (Google: Financial Services Design for High Availability Designing a Multicast Trading Floor)
• Procket documentation that briefly mentions several applications on p. 2 [Procket web site] [local copy]
• Content Delivery Networks (CDN)
• Signs showing that it might get wider use in future:
• There are a number of ISPs that enable multicast, and it is often enabled within Enterprises.  Note that most of the applications above are completely within a single enterprise network, or perhaps completely within one backbone ISPs network (i.e. CDN).
  
• But there are plenty of hurdles:
  
• The multicast routing protocols are typically not configured to work across different routing domains.  PIM-SSM is simpler in this regard, as it doesn't require the MSDP protocol for sources to become known.  PIM-SSM is really best for well known multicast servers, whose IP addresses can become known via being listed on the web (for example), or even more conveniently for the user by embedding it in a special URL. PIM-SSM requires either the end host to have IGMP version 3 implemented, or one of several hacks to work around a host that doesn't.
  
• Multicast is one more thing that can easily be forgotten to make work when it comes to configuring NAT devices, firewalls, and VPN access. Forgotten, or very justifiably ($) made much lower priority to develop and test than unicast.
• I have heard that many "tier 1" ISPs (i.e. the folks that provide broadband and cable modem access) tend not to enable it in their network because it isn't asked for much, and it costs money to train people to set it up and operate it.
• Individual multicast application flows create unique state within routers.  Network operators don't want enabling multicast to leave them open to easy denial of service attacks by hosts joining many multicast groups and consuming memory in routers.  Many router vendors are introducing configuration knobs to limit the maximum amount of multicast state that can be created by one host/interface/etc.