You can use this service to see who has registered a specific domain name for any international domain (.com, .net, etc). The "Who Is" information tells you who owns the domain, when it is due for renewal, contact info, where the domain is hosted, and so on.

Note: if you're looking for Australian domains only, use the Aus Registry site.

The Domain Tools site also includes other services, such as Domain Search, which allows you to do a wildcard search of all current, deleted, and expired domain names for over 9 years worth of history.