<form action="#">
</form>

<%= form_tag "#" do %>
    <%= label_tag :username, "Username: " %>
    <%= text_field_tag :username %>
    <br />
    <%= label_tag :password, "Password: " %>
    <%= password_field_tag :password %>
    <br />
    <%= submit_tag "Login" %>
<% end %>

:action => "#"

<%= button_tag "Login", :type => 'button'%>

<% form_tag "#", :onSubmit => "return false" do %>

<%= form_tag "" do %>
   ...
<% end %>