Zeitgeist is a really really bad documentary. It cites no sources and literally makes things up (like saying Christ was based on Horus, news to the thousands of secular and religious Jesus scholars).

Here's a really good debunking:

http://crossexamined.org/whats-wrong-with-the-zeitgeist-movie/ (A christian site, but the debunking is original from a skeptical site that has since gone bust and that was the only place that still had it)

EDIT Skeptic Project has a much better point by point rebuttal, I thought people would appreciate a more scientific host: http://skepticproject.com/articles/zeitgeist/