Acknowledgments

Peter Dimov suggested the name 'optional', and was the first to point out the need for aligned storage.
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman coded 'aligned_storage', which are the core of the optional class implementation.
Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques and their work influenced the current implementation.
Gennadiy Rozental made extensive and important comments which shaped the design.
Vesa Karvonen and Douglas Gregor made quite useful comparisons between optional, variant and any; and made other relevant comments.
Douglas Gregor and Peter Dimov commented on comparisons and evaluation in boolean contexts.
Eric Friedman helped understand the issues involved with aligned storage, move/copy operations and exception safety.
Many others have participated with useful comments: Aleksey Gurotov, Kevlin Henney, David Abrahams, and others I can't recall.

William Kempf carefully considered the originally proposed interface and suggested the new interface which is currently used. He also started and fueled the discussion about the analogy optional<>/smart pointer and about relational operators.
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson focused on the relational semantics of optional (originally undefined); concluding with the fact that the pointer-like interface doesn't make it a pointer so it shall have deep relational operators.
Augustus Saunders also explored the different relational semantics between optional<> and a pointer and developed the OptionalPointee concept as an aid against potential conflicts on generic code.
Joel de Guzman noticed that optional<> can be seen as an API on top of variant<T,nil_t>.
Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>: the Maybe type constructor (analogy originally pointed out by David Sankel).
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey, Rob Stewart, and others.
Joel de Guzman made the case for the support of references and helped with the proper semantics.
Mat Marcus shown the virtues of a value-oriented interface, influencing the current design, and contributed the idea of "none".

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