If you want to decouple validations from model it would be probably the easiest solution

simple rails html validator

Rails imei validator based on Luhn algorithm.

a class to validate ip. it builded for rails

Provides a DSL for common validation formats like Date, Array, DateTime etc.

expose presence-validation in a more controller-friendly manner

a class to validate uris. it builded for rails

Rails UseCase and Service classes

HMAC Validation for Rails.

Client Gem for Rainmaker LRO

Parser for range sentences like '1990; 1995-2000; 2005'

ActiveModel validator for ranges

Model-wrappers to customize ransack search

Services for Ruby on Rails.

Code that understands code and can change it programmatically

CASL Assembler / Simulator

A RBS files generator for activemodel gem

Framework to provide cqrs + eventsourcing functionality in ruby.

enable domain driven design approach via POROs and Repositories vs Rails' ActiveRecord

RDF for SQLite is a container gem for getting the simplest RDF-based applications off the ground. It uses quite a few modular gems and fixes a little inconsistency between RDF.rb and ActiveModel. This basically boils down to RDF.rb + ActiveModel + Spira + SQLite3 to create a pretty straightforward repository, serializer, parser, and ORM.