Most of the alternating row implementations I've seen use two css classes, "even" and "odd". However, by simply provide *one* of these, the same functionality can be achieved, and with less code.

In my application_helper.rb, I added the following:

   1  
   2    def even_odd(tag_name, options = {}, open = true)
   3      @even_odd_counters = Hash.new(false) unless defined?(@even_odd_counters)
   4      @even_odd_counters[tag_name] = !@even_odd_counters[tag_name]
   5      ((options['class'] ||= '') << ' odd').strip! if @even_odd_counters[tag_name]
   6      tag(tag_name, options, open)
   7    end
   8  
   9    def even_odd_reset(tag_name)
  10      @even_odd_counters = Hash.new(false) unless defined?(@even_odd_counters)
  11      @even_odd_counters[tag_name] = false
  12    end

This implementation uses the tag() method in Rails, so you simply pass it the same arguments. One difference, however, is that the "open" parameter is true by default, as the majority of the time this will be used for TR's (and I use it for DT and DD sometimes).

I have included a simple "reset" method as well. I realize I'm duplicating the first line in both methods, but this was a quick-and-dirty addition.