That's very hard to say. First of all, it would depend on the languages one already knows - I could make a recommendation, but that's not very useful if you already know the language.

But to give a few ideas. Knowledge of C is always useful. Even if you'll never code a single line of C, knowledge of C (and its standard libraries) makes one understand Perl better. It'll make one understand better why Perl is as it is (there are quite a number of core Perl functions that are just a thin layer above the underlaying C infrastructure). It make one understand Unix better as well.

Knowledge of SQL is useful as well. Many applications (both Web and non-Web) use a relational database; SQL knowledge is almost essential to retrieve data. But even if you're more of an administrator person is knowledge of SQL handy - it will improve one ability to troubleshoot and fix problems. Furthermore, since SQL is based on relational algebra, it exposes you to a very different way of programming than procedural, functional, or object-oriented languages do.

But in general, what to learn next depends on which direction you want to go in. Domain specific languages may be very useful if you get to use them. Learning them won't be of much use if you're not heading in a direction where they are used.

Perhaps you want job-security. Then learn a hardly used language - it may take you a while to get a job using it, but once in, you're hard to replace. Or you could go the other way, learn Java. Tons of jobs, but also a gazillion of other programmers with the same school. Useful if you want to be a job hopper.

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