What: IPv6, the latest version of the network layer Internet Protocol
Why you should care: Because IPv4 addresses are being exhausted, with total depletion expected by anywhere from 2009 to 2011, depending on whose estimate you believe. IPv6 has faced some scrutiny in carrier and user adoption, but its the most viable successor to IPv4. It's benefits include longer addresses capabilities, better security and improved auto configuration.
Who's using it: All IP backbone operators will eventually have to support IPv6. Some, like Verizon Communications, may support dual IPv6/IPv4 stacks.