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.

Description: "Our innovators need people to use their products to give them feedback! It is hard enough to develop a product. Let's make it easier for them to recruit the critical early users that they need to tell them how to make their products better." r/alphaandbetausers has 14K members and is among the top 6% of subreddits by size.