For separating a list of categories with a comma that are linked. There may be a better way. Leave a comment if you have a better way. This was the first way I found that worked.

This code increments the count 1 at a time. It applies the increment to the next loop of data.

<% category_count = @post.categories.length %><% count = 1 %>
<% @post.categories.each do |cat| %>
  <%= link_to cat.name, cat %><%= ", " unless category_count == count %><% count += 1 %>
<% end %>