Lots of people talk about "beautiful code" as if code can be art, but just as with other forms of art it can be very difficult to agree on what makes the most beautiful code. This is, therefore, a pattern full of subjectivity.
Nonetheless, I documented some instances of beautiful code according to various sources, including the Feed Validator, anything by Daniel Bernstein, emacs, TeX, Scheme, the VMS Operating System, MUMPS (included in the Portland Pattern Repository as a joke?), Zork, Lisp, nxml-mode, the Exile map generator, Ken Thompson's C backdoor, and Mark J. Dominus's Regex.pm.
Compare PPR's SoftwareMasterpiece page, and QualityWithoutaName (QWAN).