Finger trees: a simple general-purpose data structure by Ralf Hinze and Ross Paterson.
Abstract:
We introduce 2-3 finger trees, a functional representation of a persistent sequences supporting access to the ends in amortized constant time, and concatenation and splitting in time logarithmic in the size of the smaller piece. Representations achieving these bounds have appeared previously, but 2-3 finger trees are much simpler, as are the operations on them. Further, by defi ning the split operation in a general form, we obtain a general purpose data structure that can serve as a sequence, priority queue, search tree, priority search queue and more.
Before the Hinze and Paterson article you may want to read: 2-3 finger trees in ASCII by Jens Nicolay.
Note 2-3 finger trees go unmentioned in Purely Functional Data Structures by Chris Okasaki.
Other omissions of note?