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Extends Ruby's semantics by accessing its internals from pure Ruby code.,

Neo4j is a graph database for JRuby.
It supports: transactions, querying/indexing, traversal of relationships, mapping of ruby objects to a graph.
It uses the java libs neo4j and lucene.

Distributed, content-addressable storage system with repository backends for Memcached, SFTP, Amazon S3, etc.

Guard is a command line tool to easily handle events on file system modifications.

This is especially useful for automatically rerunning test cases, rebuilding and so on simply by saving a source file.

Resque is a Redis-backed Ruby library for creating background jobs, placing those jobs on multiple queues, and processing them later.

Crackup is a pretty simple, pretty secure remote backup solution for folks who want to keep their data securely backed up but aren't particularly concerned about bandwidth usage.

Canvas is an LMS project from Instructure,

Grit is a Ruby library for extracting information from a Git repository in an object oriented manner.

Goliath is an open source version of the non-blocking (asynchronous) Ruby web server framework powering PostRank. It is a lightweight framework designed to meet the following goals: bare metal performance, Rack API and middleware support, simple configuration, fully asynchronous processing, and readable and maintainable code (read: no callbacks)....