deep6
No dependency mini-library: unification, deep equivalence, deep cloning.
mergician
Uniquely flexible and light-weight utility for cloning and deep (recursive) merging of JavaScript objects. Supports descriptor values, accessor functions, and custom prototypes. Provides advanced options for customizing the clone/merge process.
tree-kit
Tree utilities which provides a full-featured extend and object-cloning facility, and various tools to deal with nested object structures.
js-deep-clone
A JavaScript module for deep cloning objects and arrays, with support for dates, while handling circular references.