Using social media and relevant communities!
At least that's what worked for us - we've just launched our product Reply: Email Search which became the #1 product of the day (yay!) mostly thanks to the support of the sales community.
Maybe that was due to the perfect timing (the sales downturn most businesses are experiencing right now) and its tangible value, the purpose (to help sales teams generate businesses despite the crisis), or just because it's free - the support has been amazing!
Also, our business partners have been very supportive too, spreading the news among their users and social following!