By default, /r/reddit.com is added to everyone's subscription list. It's sort of a "catch-all" reddit that you can submit any content to.
A lot of people end up unsubscribing to it though, because it's overrun by the hoi polloi.
There are some other "any topic is allowed" subreddits, which, rather than focusing on a specific TOPIC, instead focus on specific EDITORIAL GUIDELINES.