157 Transparent Fluorescent Yellow / Trans-Neon Yellow
BrickLink: Trans-Neon Yellow
Peeron: TrPeach (yes, really)
Unlike Transparent Yellow (which is much more common), this color glows brightly under a blacklight. Used from 2001-2005, in a very limited selection of parts. Bionicle brain stalks (left in photo) are the most common; the KauKau (center) was a "misprint" that appeared in European Kanohi packs. Clikits used this color occasionally, and one Belville set (5960 Mermaid Castle) had 1x1 cones and round plates in this color.
Glows brightly under a UV light.