who.is is a simple web-based WHOIS gateway, supporting pretty much all top-level domains. In case you didn’t know, WHOIS is a mechanism of finding out who (if anyone) has registered a particular name. It’s fairly quick, has a nice clean design and negates the need to go to separate sites for different domains.
There are alternatives out there but the short, memorable URL and the ability to do quick lookups using only the address bar (e.g. who.is/chrisbarnes.net) makes it my favourite by far.