BetaList is a large and well-known startup directory but it's also very selective. They review each submission before they decide whether to allow it on the platform. Your startup should be pre-launch or recently launched without substantial press coverage, needs a custom designed, decent-looking landing describing the product and a way for people to sign up.

r/startups is a subreddit for everything related to startups. Here is how they describe themselves: The community for ventures designed to scale rapidly. Welcome to /r/startups, the place to discuss startup problems and solutions. Startups are companies that are designed to grow and scale rapidly. r/startups has 1.6 million members and is among the top 1% of subreddits by size.