As others have mentioned, it is, and it's also airing irregularly on the Sci-Fi channel. After the series was cancelled, creator Joss Whedon (the same guy behind Buffy and Angel) wrapped up the story line with a theatrical movie, Serenity.
One of my favorite things about Shepherd Book is that he seems to know all about government black ops, for reasons that are never explained. The closest they get to explaining that:
Shepherd Book: I wasn't born a shepherd, Mal.
Capt. Malcolm Reynolds: You have to tell me about that sometime.
Shepherd Book: [pause] No, I don't.
I've just got to see this series. Thanks for the info.