Simple Ruby Input Validation Library

You could use ActiveModel::Validations, from Rails 3 RC:

require 'active_model'
# this appears to be a bug in ActiveModel - it uses this, but does not require it
require 'active_support/core_ext/hash'

class Model
  include ActiveModel::Validations

  attr_accessor :name
  validates_presence_of :name
end

m = model.new
puts m.valid? # false
m.name = "John Doe"
puts m.valid? # true