That's an IPv6 address. Someone is connecting from a host with an IPv6 address, rather than one with the usual IPv4 addresses you're probably accustomed to.
To find out where an IPv4 or IPv6 address is located, you can start with querying WHOIS.ARIN.NET. If it's not in North America, you will see a reference to the appropriate network numbers registrar.
ARIN.Net (see WHOIS query input box in top-right corner)
Wikipedia provides a good introduction to IPv6:
Wikipedia - IPv6
Additionally, for more technical information about IPv6, one very helpful resource is the #IPv6 channel on the FreeNode.Net IRC network:
FreeNode.Net web-based IRC client on the IPv6 channel