sqlmap is a penetration testing tool that automates the process of detecting and exploiting SQL injection flaws and taking over of database servers. It comes with a detection engine, many niche features, and a broad range of switches including database fingerprinting, data fetching from the database, and accessing the underlying file system and executing commands on the operating system via out-of-band connections.
nessus-xmlrpc is a Ruby library for the Nessus XML-RPC interface. You can use it to start, stop, pause, and resume scans. It lets you watch progress and status of scans, download reports, etc. It comes with an example command line program that shows how easy it is to interact with the Nessus scanner.
RubyDNS is a high-performance DNS server that can be easily integrated into other projects or used as a stand-alone daemon (via RExec). By default, it uses rule-based pattern matching. Results can be hard-coded, computed, fetched from a remote DNS server, or fetched from a local cache, depending on requirements. In addition, RubyDNS includes a high-performance asynchronous DNS resolver built on top of EventMachine. This module can be used by itself in client applications without using the full RubyDNS server stack.