Domain Name Analyzer is freeware for Windows and MacOSX that allows you to not only find out if a domain name is taken, but it can find out when it expires and whether or not a domain is available.
What’s more is that you can also set up names and have it generate names that might be good choices for domains. It’s just an all around domain checker type application that you can save and even register from your favorite registrar.
To me, it’s useful because it’s tough enough to type in different types of names into one of those registrars and have to wait for a website to pull up the data. Instead, this application goes directly to the whois databases and queries those. Ahh. What a life changer.