With ActsAsTaggableOn, you can tag a single model on several contexts, such as skills, interests, and awards. It also provides other advanced functionality.

A good, simple, solid tagging extension for ActiveRecord

tagging that doesn't want to be on steroids. it's skinny and happy to stay that way.

Rails plugin that is based on acts_as_taggable by DHH but includes extras such as tests, smarter tag assignment, and tag cloud calculations.

Adds tagging with context and aggregation of tags for MongoMapper. Adds weight and distribution convenience methods to models in which it is included. Uses MongoDB's increment/decrement ($inc/$dec) to keep real-time counts of individual tags in context with optional type. Based on 'Mongoid Taggab...

Simple tagging gem for Rails using postgres array.

Clortho makes keyword and full-text search a breeze for MongoMapper users.

Markable allows you to easily create a marking system in your rails application.

A modern fast tagging framework for Rails 3.1+

Allows user owned tags for multiple classes, and makes tags easier to work with.