Just Search YouTube web widget "Just Search YouTube" is a little YouTube player by sgtvideo with a searchbox on top! You can search your videos by entering keywords or just put the video id into the searchbox. Have fun with this little playerwidget. it's a good size for on your google personal page or blog. Love, Sgt video dot com
FOX: FOX is currently streaming full episodes of some of its programs in select markets. Unfortunately, you can only watch these episodes if you live within that specific broadcast area. Right now those areas include Dallas, Boston, Greensboro, Los Angeles, New York, Birmingham, Tampa Bay, Orlando and Washington, DC. FOX also offers $1.99 downloads on the IGN Direct2Drive site. FOX has deals with several sites to stream its television shows, including the News Corp-owned MySpace, plus Google Video, MSN, and TV Guide.
AOL Video also offers a surfeit of shows both for free and at a cost (thanks, Rod).
Yahoo TV offers some (but not many) full episodes from the major networks, as well: http://tv.yahoo.com/
MSN: Premieres offered for free on the TV Buzz section of the MSN site.
Amazon Unbox also has shows from several networks available for a cost.
I'm not going to list specific TV shows but rather provide links to the download sites since TV shows come and go and new ones are always coming down the pike. I figure this way you can stick this post in your Favorites and come back to it whenever you feel like foraging for TV content online. As of this writing, the following networks are offering content through the iTunes store: A&E, ABC, ABC Family, ABC News, Adult Swim, Animal Planet, Biography Channel, Bravo, Cartoon Network, CBS, CNN, Comedy Central, CSTV/CBS Sports, Discovery Channel, Discovery Health, Discovery Kids, Disney Channel, E!, ESPN, FOX, FOX Reality, Fuel TV, FX, History Channel, FX, Jetix, Logo, MTV, NBC, NBC News, NFL Network, Nick at Nite, Nickelodeon, Sci Fi Channel, Showtime, Sleuth, SOAPNet, Speed, Spike, The N, TLC, Travel Channel, TV Land, USA and Warner Bros.
TV shows can also be found on Google Video if you feel like poking around. I personally find the site to be a bit too arduous for my taste. UPDATE: Reader Lampbane made a list of the shows offered through Google in thecomments. Thanks, Lamp.
If I missed anything, and I probably did, let me know in the comments. Keep in mind I'm only interested in full episodes, not clips.
NEW: Commenter dr.tv has a massive list of shows available online here. A great source.