Free DNS services are designed to speed up your browsing, but just because Google or OpenDns wants to be fast doesn't actually mean they're the best option for you. Namebench finds the fastest DNS server for your connection. After you download and fire up namebench, just click the Start Benchmark button to test out a handful of free public DNS services, from Google Public DNS to OpenDNS to UltraDNS. Once started, the test took about 12 minutes to complete using the default settings. When it's done, you'll be presented with a handful of handy charts displaying the results of the tests.
Download from here http://code.google.com/p/namebench/