Are you looking to build a WMS server or for suggestions on where to find existing WMS services? In general, WMS isn't that great-- there are few (or none?) commercially reliable services to use. Most tend to be government, academic and non profit and focus on specific layer types and a limited geography. The spec was also last updated over 10 years ago, so there's just not a lot going on in WMS, but the this thread might be helpful. Another approach is to look at the enhanced mapping capabilities through Urban Mapping's technology (what powers Tableau mapping). You can also get an eval to play around with many more layers.
If you want OpenStreetMaps then use http://22.214.171.124/cached/osm? . Add that to your list of WMS servers. This is hosted by the University of Heidelberg under GPL licence. I have had 99% up time in the past six months, and it's updated on a regular basis. OSM -> WMS is NOT a trivial process.
Depending on where you are in the world, there are exceptional WMS servers, let me know what you want to visualize. (25 foot High Resolution Orthoimagery for example).
I've also built my own, as per Christian's suggestion, using Geoserver, so I can run mapping offline/reduce my dependence on Tableau/other services/get loads of shapefiles into layers. Expect a blog post soon.
I am also using my own Geoserver to get round Tableau's insistence of using a set datum.
See alternate WMS in action here.
You could upvote my idea; "crowd sourced WMS server"