Windows only: Freeware program TVUplayer lets you watch potentially any TV channel for free.

TVUplayer works like this: Someone (called a “broadcaster”) records live TV on their computer, then sends out the stream to the world at large. From that point, the more people who decide to watch the station, the better the “signal” you’ll receive, since TVUplayer uses peers to redistribute content similar to bittorrent (or that’s how I understand it). That means it’s going to be uploading plenty while it’s running (and believe me, it did). What I was most impressed by is that TVUplayer was less than 1 minute behind my live TV, which is pretty impressive.
The quality was good, especially for streaming video, but this will depend on the channel you watch. If you try a station with fewer viewers, the signal won’t be as strong, and you stream will suffer.

Will this last? Probably not. But if you’re looking for yet another good way to watch some free TV on your computer (with channels like ESPN, HBO, Comedy Central, and all the major networks), TVUplayer is a good place to look.