Because it is the closest port to mainland Britain with just 21 miles from the British port of Dover, Calais is one of the busiest cross-Channel ports on continental Europe. Regular ferry services operate from Calais to Dover (P&O and Seafrance each with over 20 crossings per day). Dunkirk could be an alternative to get over (norfolklines to Dover, same or nearly same conditions than Calais – Dover), Calais is busier though.
Hitchhikers can get over for free because drivers only pay for their vehicle and up to nine persons go for free with a car and two with a lorry – additional persons pay like footpassengers. If you are searching for a lift you often have to tell people about this possibility first – otherwise they often don't take you with them just because they think it's too complicated or they'll have to pay...
Lorry Park with great possibilities Edit
Near Calais on the motorway, few kilometres from the ferryport, there is a huge lorry park from where you can easily get direct lifts to many places all over Europe (Belgium, Netherlands, England, Germany, Poland, Spain, Czech Republic, and Austria should be easily possible).
There is also a service station with free showers.