OKCUPID is a free dating site: Everything you need (email, chat, search,...) is free. It follows a freemium model where you can pay if you want some non-essential extra features (e.g., visiting profiles without being listed as visitor and fine-grained search).
There are more people on the site than on many other sites and the people usually respond to emails (I'd say I get responses to 50% of my emails while at plentyoffish this rate is more like 5%)
Overall, my favourite and totally free dating site.