C and Lisp. Nearly every hacker knows one of these, and most good ones are fluent in both. Smalltalk and Prolog are also popular in small but influential communities.
There is also a rapidly dwindling category of older hackers with Fortran, or even assembler, as their language of choice. They often prefer to be known as Real Programmers, and other hackers consider them a bit odd (see "The Story of Mel"). Assembler is generally no longer considered interesting or appropriate for anything but HLL implementation, glue, and a few time-critical and hardware-specific uses in systems programs. Fortran occupies a shrinking niche in scientific programming.
Most hackers tend to frown on languages like Pascal and Ada, which don't give them the near-total freedom considered necessary for hacking (see bondage-and-discipline language), and to regard everything even remotely connected with COBOL or other traditional card walloper languages as a total and unmitigated loss.
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Nearby terms: Language Of Temporal Ordering Specification « Language Sensitive Editor « language-sensitive editor « languages of choice » Laning and Zierler » Lan Kanal Adapter » LANL