46,136 subscribers a community for 9 years, 6 monthslast post today [+]
/r/serendipity is a meta-subreddit meant to broaden the perspective of its subscribers. It takes a popular entry from a random subreddit and posts it every few hours.
If you want to increase your exposure to niche subreddits, or just your perspective on things on the web in general, serendipity might help you do that. But it might not. It's a bot, after all.
If you like Serendipity, you might also like /r/subredditoftheday, a community of curated subreddits and interviews with the people who created the, or this list of curated discovery subreddits in /r/DiscoverReddit.