SPM's Containers Library
This library contains odds and ends that I have found interesting over time.
2-3 Finger Tree
My implementation, based on the 2006 Hinze and Paterson paper, provides a fully-persistent data structure suitable for implementing other data structures such as indexable sequences, catenable deques, and priority queues. The Haskell programming language uses the 2-3 finger tree as the underlying implementation of Data.Sequence.
Generally, 2-3 finger trees provide good worst-case asymptotic behavior for all operations at the cost of high constant factors.
Implementing a data structure atop a 2-3 finger tree entails providing a monoid and a measurement function to suit your use case. For example, an indexable sequence of elements of type A requires the monoid (Z, +) (i.e., the set of integers and the addition operation) and the measurement function f(x) = 1, where f : A -> Z (i.e., a unary function that always returns one).