Does the Estonian Startup Visa let me travel in the Schengen area?

Because the Startup Visa is a long-stay (D) visa, it does more than authorise your stay in Estonia. During its validity it functions for Schengen travel, letting you make short visits to other Schengen countries within the standard 90-days-in-180 short-stay rules.

This is useful for founders attending events, meeting investors, or visiting partners elsewhere in Europe. Keep within the short-stay limits in other countries, since the visa's main purpose is your startup activity based in Estonia.