My DSL provider is pretty good in some ways, but the DNS service has some drawbacks:

(i) It used to be ~slow~, slow enough that my broadband connection felt worse than dialup, (ii) Failed DNS queries get pointed to a server that serves up contextual ads.

The second is obnoxious when you're browsing the web, but it's completely unacceptable when you're doing projects where you expect certain DNS queries to fail.

djb's dnscache is a sweet answer to the problem.