Organisations erecting plaques
Popular opinion in the UK is that English Heritage are the only organisation that can create 'official' plaques. This is totally wrong. There are a multitude of organisations that create plaques ranging from local authorities to specialist fan groups to individuals. Anybody can have a plaque created, the main limiting factor being cost (a couple of hundred pounds for a good cast metal plaque that will last) and having the freeholder's permission. In other countries the laws will be different, for example each US State will often have a historical markers board, but it still appears that multiple groups are active.
OpenPlaques has 4517 organisations registered.