You can always use the major PPC advertisers like Google, MSN, and Yahoo but they can end up costing you way more than you would like to spend. Lucky for us there are places out there offering free pay per click. These are older PPC engines but they can still bring traffic to your site. Here is a list below of sites that offer free PPC sign on bonuses:
www.searchpost.net/list.htm $500 Worth Of FREE Advertising!
I am sure you could find more if you keep searching.