link_to

rubyonrails-talk
1

NW, here is the solution.

<%= link_to trip.user.username, { :controller => :page, :action => :sendmail, :username => trip.user.username }, :popup => true %>

2

or

<%= link_to trip.user.username, { :controller => :page, :action => :sendmail, :username => trip.user.username }, :popup => ['Contact User', 'height=600,width=800'] %>