Domain Checker is a desktop whois tool which can check the availability of one, a few or hundreds of domains.
Checking a single domain is easy. Type it in the Domain Name box, press Enter, click Check All, and within a moment you'll see whether it's registered or not.
Right-click that domain and you can view its details or visit the site.
The program can also generate and check 2, 3, 4 or multiple character domain names. If you're unsure what to call your software site, for instance, you could define a base address (-soft.com), generate a sequence of names (aa-soft.com, ab-soft.com...), and check them all to see which are available.
There are also tools to generate domain names by user formula, importing words from text files, and more.
Domain Checker need some work-- its interface is awkward, and one feature ("Find Related Words") didn't work for us at all. But it's not difficult to use, and even now there's enough domain-checking functionality to be useful.