United Kingdom/Editing Resources
(courtesy of Timbones' Forum Topic)
(Of course, we're not allowed to copy directly from any 3rd-party maps or aerials, but they can be used for research.)
http://www.google.co.uk/maps (including Streetview)
http://www.bing.co.uk/maps (also includes OS maps for some zoom levels)
http://www.openstreetmap.com (possibly more up-to-date, but don't rely on it)
http://www.trafficengland.com (also Trunk roads)
These can be useful when trying to figure out the names of junctions, restrictions and classifications.
http://www.cbrd.co.uk/motorway/ (Motorway database, and other good stuff. Not always up-to-date)
http://www.sabre-roads.org.uk/wiki/index.php?title=Main_Page (Wiki of UK roads)
http://en.wikipedia.org/wiki/Category:Roads_in_the_United_Kingdom (Wikipedia - duh!)
http://www.roadsuk.com Loads of information about UK roads, including history, road numbering, signage, etc.
http://www.pathetic.org.uk A great site about British Motorways, their history, secret ones, lost ones and unfinished ones. Worth a read.
New Roads and Junctions
When mapping new roads or junction layouts, the big online map sites are often months or years behind. These resources can help you figure out what's what:
http://www.highways.gov.uk/roads/29639.aspx (Highways Agency road projects - other sites for Scotland and Wales)
http://www.sabre-roads.org.uk/forum/ (Sabre forum - often has threads about specific projects)
http://www.google.co.uk/maps (Streetview sometimes has more recent pictures than you see on the map, especially when a new road is constructed an old one. Useful for working out new junctions)