Default_textfield_that_clears_on_click.rb

# application.helper
def default_text(text)  
   onClickFunction = "field = event.target; if (field.value=='#{text}') {field.value = '';}else {return false}"  
   onBlurFunction = "field = event.target; if (field.value=='') {field.value = '#{text}';}else {return false}"  
         {:value => text, :onclick => onClickFunction, :onblur => onBlurFunction}   
end 

# view

<%= text_field_tag 'login', @login , default_text("Login")%>