Phil Bagwell's VList is a really cute data structure. It's an immutable list with O(1) cons and cdr, lookups that are expected constant time and O(lg n) worst-case time, and better cache locality than linked lists. The wikipedia entry is short and understandable, which makes it easy to honor the dead in a pleasant and enlightening way:

http://en.wikipedia.org/wiki/VList

Obviously a very clever guy.