Großbritannien:

Easy Roommate -Flatshare:
http://uk.easyroommate.com/

Flatshare:
http://www.flatshare.com/

Gumtree:
https://www.gumtree.com/all/uk/room

Room Buddies:
https://www.roombuddies.co.uk/

SpareRoom:
http://www.spareroom.co.uk/