<a href="http://video.google.com">Google Video</a>
Duh
<a href="http://www.turbonick.com">TurboNick</a>
Nickelodeon shows and clips online. Right now I'm watching Rocko's Modern Life full-screen on it.
<a href="http://www.adultswim.com/adultswimfix">Adult Swim Fix</a>
Recently they expanded their lineup and put up many more episodes per week for viewing. I can't connect to them with anything besides Windows Media Player (so no VLC), but that's OK. If you use Firefox you can easily get the "mms://" link strings out of the .asx files so you can directly view the episodes at your leisure.
<a href="http://music.yahoo.com">Yahoo! Music (formerly LAUNCHcast)</a>
Tons of music videos and internet radio. This is good because I have the PC connected to the stereo reciever.
And of course I use VLC to stream DVDs and other video files to it.
With most of these sites, when the image is adjusted right it can be hard to tell you're watching a media file on a PC.
Hopefully in the future, when I have some free cash, I can build a better setup. (right now it's an old Compaq small form factor PC with integrated graphics and an external PC-TV bridge that's probably as old as some of you younger kids here.) Who knows; it may get built into the cabinet with a cheap DVD player holding the optical drive and buttons to control MediaPortal. (A Windows program that imitates Microsoft's Media Center software)
Anyways, does anyone know of any other good streaming media sites? (preferably free because... free is good.