(Note that these are mainly restricted to requests from internet addresses in their own country.)
BBC "iplayer": current, live, bigscreen and archives (UK restricted):
http://www.bbc.co.uk/iplayer/tv
http://www.bbc.co.uk/bbcone/watchlive
http://www.bbc.co.uk/bbctwo/watchlive
http://www.bbc.co.uk/iplayer/bigscreen/
http://www.bbc.co.uk/archive/collections.shtml
ITV (UK restricted)
http://www.itv.com/itvplayer/
http://www.itv.com/itv1/
http://www.itv.com/itv2/
http://www.itv.com/itv3/
http://www.itv.com/itv4/
Channel4 & friends
http://www.channel4.com/programmes
Channel 5
http://www.channel5.com/demand5
sky: UK-favouring [reduced size/content elsewhere], and uses Silverlight instead of Flash
http://www.adtelly.tv/sky_news.html
UK-centric tv
http://player.stv.tv/ (not geographically restricted)
http://www.u.tv/utvplayer/ (geographically restricted)
NBC not geographically restricted; continuous adverts mitigated by Firefox AdBlockVideo addon
http://www.nbc.com/video/library/
CBS not geographically restricted
http://www.cbs.com/video/
ABC (US restricted): rubbish site -- player just says "loading"
http://abc.go.com/watch
HULU: content from ABC, but player at least works (though still US-restricted)
http://www.hulu.com/
FOX (US restricted)
http://www.fox.com/
Other US-based channels (haven't tested all for restriction)
http://dsc.discovery.com/
http://cwtv.com/cw-video/
http://www.aetv.com/videos
http://www.history.com/shows
http://disney.go.com/videos/#/videos/tvshows/
http://www.cnn.com/video
http://www.accesshollywood.com/
http://www.thewb.com/shows/full-episodes
http://www.kidswb.com/video
http://www.nick.com/videos/
http://www.nickjr.com/kids-videos/
http://www.syfy.com/
http://www.spike.com/full-episodes
http://www.vh1.com/video/#episodes-carousel
http://www.hgtv.com/full-episodes/
http://video.pbs.org/
http://www.tvland.com/full-episodes
http://www.tntdrama.com/
http://www.eonline.com/videos/index.jsp
http://www.qvc.com/onair
http://www.adultswim.com/