What exactly are you looking for aside from being able to watch Discovery, History and Fox News? You know, you weren't very clear on that. You can watch their content from their individual websites, but if you're looking a "Media Center" experience, that might be harder to come by.
The thread dav234 linked to contains a fairly extensive list of online content providers.
I personally use Netflix. They've got a growing number of titles available on Instant Watch. Whatever they don't have on Instant Watch, I just get via mail. For the Instant Watch functionality, I got a Roku Netflix Player for my mom. It's very intuitive and it just works.
I also have a TV tuner for broadcast stuff, but I really don't watch a lot of TV.