While helping out a colleague yesterday I discovered that many of the WMS servers that I have used in the past are now offline. This also seems to apply to many of the “how to use a WMS in QGis” posts that Google returns too.

So after some searching and poking around in old archives I chanced across USGS’s The National Map - Service Endpoints which helpfully provides some useful endpoints. Most of the layers have a link (labeled WMS) at the end which you can copy directly in to the QGis WMS connection box.