Bundler-Audit - Provides patch-level verification for Bundled apps
Codesake::Dawn - Dawn is a static analysis security scanner for ruby written web applications. It supports Sinatra, Padrino and Ruby on Rails frameworks
Service Interaction
PwnedCheck - Checks an email address to see if it’s found on http://haveibeenpwned.com
Secure Coding
Loofah - General library for manipulating and transforming HTML/XML documents and fragments
Shellcode / Assembly
Crabstone - Ruby binding to the capstone disassembly library
FFI-udis86 - Provides Ruby FFI bindings for the udis86, a x86 and x86-64 disassembler
Metasm - Ruby assembly manipulation suite (now included in Metasploit)
Ronin-asm - Ruby DSL for crafting Assembly programs and Shellcode
Spidering
Anemone - Ruby library that makes it quick and painless to write programs that spider a website
Spidr - Ruby web spidering library that can spider a site, multiple domains, certain links or infinitely
Tarantula - Crawls your Rails 2.3 and 3.x applications, fuzzing data to see what breaks
Guides to securing ruby code / configuration / links / etc.