You can watch recent 24 episodes online in HD by going here:
You can also navigate to this site by going to www.fox.com
, selecting the show you are interested in, and then clicking on a link to "Fox on Demand"
All the major networks have some shows available online. I have tried ABC, NBC, CBS, and Fox's streaming viewers.
The Fox viewer is by far the best (highest resolution, no pauses/delays). It is the only one that requires you to click to accept installation of their streaming video viewer software, and it appears to be hosted through MySpace. Maybe they have better server bandwidth or fewer users? I installed and have been using it with the latest Windows Explorer browser.