- rake - build tool
- rack - http
- thor - build tool
- nokogiri - xml, html, sax processor
- arel - SQL AST
- minitest - testing
- thread_safe - threading
- daemons - wrapping scripts as daemons
- hike - file operations
- treetop - expression grammar (PEG) for DSL
- rest-client - REST
- eventmachine - single thread for network communication
- highline - IO for cli (validation etc.)
- pry - irb alternative and debugger
- redis-rb - redis client
- hashie - additions to Hashes
- haml - HAML
- httparty - http
- childprocess - processes
- watir - selenium lib
- passive_record - in-memory pseudo-relational data structures
- dotenv - configs
- guard - tasks on file changes
- resque - redis backend for background jobs