DISCLAIMER: At the time when you'll read this, you can expect some dead links !

GeoServer:
http://sigma.openplans.org/geoserver/ows (dead link)
http://demo.opengeo.org/geoserver/wfs (WFS-T with write permissions)
http://giswebservices.massgis.state.ma.us/geoserver/wfs
http://gem.demo.opengeo.org/geoserver/wfs
http://geohub.jrc.ec.europa.eu/effis/ows
http://v2.suite.opengeo.org/geoserver/ows
http://data.wien.gv.at/daten/wfs  # layers with identical names within 2 different namespaces
http://geoserv.weichand.de:8080/geoserver/wfs?VERSION=2.0.0 (WFS 2.0 capable, with paging)
http://briseide02.ingr.briseide.eu:8080/geoserver/briseide/ows?VERSION=2.0.0 (WFS 2.0 capable, with paging)
http://gis.facilities.ucsb.edu:8080/geoserver/wfs
http://kartenn.region-bretagne.fr/geoserver/wfs
http://agismap1.faa.gov/geoserver/ows (AIXM data)

MapServer:
http://www.bsc-eoc.org/cgi-bin/bsc_ows.asp (4.8.1)
http://www2.dmsolutions.ca/cgi-bin/mswfs_gmap
http://www.rijkswaterstaat.nl/services/geoservices/overzichtskaartnl
http://devgeo.cciw.ca/cgi-bin/mapserv/carts
http://map.ns.ec.gc.ca/MapServer/mapserv.exe?map=/mapserver/services/envdat/config.map (4.8.3)
http://nsidc.org/cgi-bin/acap.pl
http://sdmdataaccess.nrcs.usda.gov/Spatial/SDMNAD83Geographic.wfs
http://demo.mapserver.org/cgi-bin/wfs?
http://hip.latuviitta.org/cgi-bin/mapserver_wfs
http://geolittoral.application.equipement.gouv.fr/wfs/metropole?VERSION=1.0.0 (5.6)

Deegree:
http://www.nokis.org/deegree2/ogcwebservice (not responding, requires namespace to be passed)
http://www.idee.es/IDEE-WFS/ogcwebservice (responding, requires namespace to be passed)
http://demo.deegree.org/deegree-wfs/services
http://testing.deegree.org/deegree-wfs/services (WFS-T, but permission denied)
http://lkee-xplanung2.lat-lon.de/xplan-wfs/services (with complex schema that we cannot understand with the XSD parser)
http://deegree3-testing.deegree.org/deegree-geosciml-demo/services (offline currently)
http://deegree3-demo.deegree.org:80/inspire-workspace/services (GML 3.2.1 output only and WFS 2.0 capable)
http://deegree3-demo.deegree.org:80/utah-workspace/services (WFS 2.0 capable)
http://inspire.kademo.nl/deegree-wfs/services
http://www.iai.fzk.de/wfs (Deegree 2)

TinyOWS:
http://dev4.mapgears.com/cgi-bin/tinyows (old & buggy version)
http://www.tinyows.org/cgi-bin/tinyows (WFS-T with write permissions)
http://gis-lab.info/scripts/tinyows/tinyows.cgi
http://hip.latuviitta.org/cgi-bin/tinyows

CubeWerx:
http://ceoware2.ccrs.nrcan.gc.ca/cubewerx/cwwfs/cubeserv.cgi?datastore=CEOWARE2
http://cgns.nrcan.gc.ca/wfs/cubeserv.cgi?DATASTORE=cgns
http://portal.cubewerx.com/cubewerx/cubeserv.cgi?DATASTORE=Foundation
http://portal.cubewerx.com/cubewerx/cubeserv.cgi?CONFIG=haiti_vgi&DATASTORE=vgi  (offline: was WFS-T with write permissions)

Intergraph:
http://regis.intergraph.com/wfs/dcmetro/request.asp
http://geonames.nga.mil/nameswfs/request.aspx (fails often)
http://ideg.xunta.es/WFS_POL/request.aspx

ESRI:
http://map.ngdc.noaa.gov/wfsconnector/com.esri.wfs.Esrimap/Sample_Index_f (no layers published)
http://map.ngdc.noaa.gov/wfsconnector/com.esri.wfs.Esrimap/dart_atlantic_f (works)
http://siga.cna.gob.mx/wfsconnector/com.esri.wfs.Esrimap/Acuiferos_Features (encoding reported as UTF-8, but ISO-8859-1 probably)
http://sentinel.ga.gov.au/wfsconnector/com.esri.wfs.Esrimap (works)
http://sima.gencat.cat/wfsconnector/com.esri.wfs.Esrimap/SIMA_WFS (returns invalid XML in GetFeature)
http://egisws01.nos.noaa.gov/wfsconnector/com.esri.wfs.Esrimap/nosdataexplorer_wfs (Mark invalid error in GetFeature)
http://services.nationalmap.gov/ArcGIS/services/geonames/MapServer/WFSServer
http://129.116.104.176/ArcGIS/services/Climate/CFSRMonthly/MapServer/WFSServer (offline)

Microimages ?:
http://publicatlas.microimages.com/tntgateway/tntgateway.cgi

Ionic:
http://webservices.ionicsoft.com/ionicweb/wfs/BOSTON_ORA (1.0.0 WFS-T)
http://webservices.ionicsoft.com/gazetteer/wfs/GNS_GAZ
http://demo.ionicsoft.com/agiRSW/wfs/PYLONS

Safe Software:
http://142.176.62.103:8194/servlet/WFS_NS_NAMES (offline)

ERDAS Apollo:
http://apollo.erdas.com/erdas-apollo/vector/Cherokee

SnowFlake software:
http://demo.snowflakesoftware.com:8080/AIXM51FINAL_DONLON/GOPublisherWFS

Mapinfo:
http://www.mapinfo.com/miwfs

???
http://services.interactive-instruments.de/xtra/cgi-bin/wfs?Request=GetCapabilities (doesn't work well)

??? (WFS 2.0)
http://services.cuzk.cz/wfs/inspire-cp-wfs.asp?

Catalog with a lot of dead links, but a few of the above actually come from here :
http://svn.geotools.org/branches/xdo-branch/plugin/wms/test/org/geotools/data/wms/test/test-data/servers.txt

Other catalog with a lot of alive links :-) :
http://ows-search-engine.appspot.com/index?st=WFS