Who Is

Find out if your domain is available, the history of ownership, and by whom a site is hosted:

Whois Lookup

There are others, easy to discover through Google, but be careful!  If you are looking to see if a particular domain is available, some domain registrar companies will place a hold on the domain if it is available which may force you to purchase the domain through them if you want the domain right away.   Not ethical, but legal.