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.

This directory boasts 20,000 subscribers and if you get featured, you get a backlink. You'll also be featured in the newsletter. Insanely Cool Tools is free to use but you may need to pay if you want to skip the queue.