(See my note on DomainSpecificLanguage for a quick intro to this topic and my terminology on it.)

Update:David Laribee has written a post contrasting what he calls ordered and unordered fluent interfaces. The distinction is that ordered fluent interfaces force a particular flow on how you compose your DSL sentence. He provides an example where he uses multiple interfaces on a single ExpressionBuilder - the same technique that's used by JMock.

Anders Norås has written two interesting articles on writing internal DSLs in C#. The first article gives a sample of the DSL and a discussion against Chromatic's cynical check-list. The second article goes into details about its implementation.

Piers Cawley makes the point that a key characteristic of DSLs is their narrow focus on a domain.