proper logging -- with color :)

log_formatting.rb

# Define a setter to pass in a custom log formatter
class ActiveSupport::BufferedLogger
  def formatter=(formatter)
    @log.formatter = formatter
  end
end

# Defines a custom log format (time, severity, message, PID, backtrace)... all with color!
class Formatter
  SEVERITY_TO_TAG   = {'DEBUG'=>'meh', 'INFO'=>'fyi', 'WARN'=>'hmm', 'ERROR'=>'wtf', 'FATAL'=>'omg', 'UNKNOWN'=>'???'}
  SEVERITY_TO_COLOR = {'DEBUG'=>'37',  'INFO'=>'32',  'WARN'=>'33',  'ERROR'=>'31',  'FATAL'=>'31',  'UNKNOWN'=>'37'}
  HUMOR_FOR_ENV     = {development: true, test: true, production: false}
  DEPTH_FOR_ENV     = {development: 3,    test: 3,    production: 1}
  EXCLUSION_REGEX   = /log|active_support|active_record/

  def humorous?
    return @is_humorous if defined? @is_humorous
    @is_humorous = HUMOR_FOR_ENV[ Rails.env.to_sym ]
  end

  def depth
    @depth ||= DEPTH_FOR_ENV[ Rails.env.to_sym ]
  end

  def call(severity, time, progname, msg)
    t     = time.strftime("%Y-%m-%d %H:%M:%S.") << time.usec.to_s[0..2].rjust(3)
    color = SEVERITY_TO_COLOR[severity]
    sev   = humorous? ? "%-3s" % SEVERITY_TO_TAG[severity] # pad to at least 3 characters
                      : "%-5s" % severity                  # pad to at least 5 characters

    # 2013-05-01 19:16:00.785 [omg] oh noes! (pid:30976) (admin/user.rb:45:in `block (4 levels) in <top (required)>') <- `call' <- `content_for' <- `block (2 levels) in row' <- `block in build_tag'
    "\033[0;37m#{t}\033[0m [\033[#{color}m#{sev}\033[0m] #{msg.strip} (pid:#{$$}) #{whodunit}\033[0m\n"
  end

  def whodunit
    latest, *others = caller.select{ |a| a !~ EXCLUSION_REGEX }[0, depth]
    latest = latest[/(lib|app)\/(.*)/,-1] || latest
    string = ""
    string << "\033[36m(#{latest})"
    string << "\033[35m <- " + others.map{ |s| s[/`.*/] }.join(' <- ') if others.any?
    string
  end

end

Rails.logger.formatter = Formatter.new

I'm on Rails 4.2.8 and I had to add:
include ActiveSupport::TaggedLogging::Formatter
to the Formatter class.