An awesome nested set implementation for Active Record
Ancestry allows the records of a ActiveRecord model to be organized in a tree structure, using a single, intuitively formatted database column. It exposes all the standard tree structure relations (ancestors, parent, root, children, siblings, descendants) and all of them can be fetched in a single sql query. Additional features are named_scopes, integrity checking, integrity restoration, arrangement of (sub)tree into hashes and different strategies for dealing with orphaned records.
Easily and efficiently make your ActiveRecord model support hierarchies
A gem that adds simple support for organizing ActiveRecord models into parent–children relationships.
better_nested_set Rails plugin (my fork from official SVN)
NOTICE: official repository moved to https://github.com/bbommarito/acts_as_nested_set
ActiveRecord extension for sorted adjacency lists support
Adds directed acyclic graph functionality to ActiveRecord.
Arboreal is yet another extension to ActiveRecord to support tree-shaped data structures. Internally, Arboreal maintains a computed "ancestry_string" column, which caches the path from the root of a tree to each node, allowing efficient retrieval of both ancestors and descendants. Arboreal surfaces relationships within the tree like "children", "ancestors", "descendants", and "siblings" as scopes, so that additional filtering/pagination can be performed.