Mailchimp & Sendgrid for mail (I like MC for campaigns, but SG for API call driven messages).

Firebase for hosting/serverless funcs.

G Suite for collaboration.

Bitbucket for Repos (they had better enterprise-y tools for free, not sure if Github now is at parity).

Notion for task lists and product specs.

Stripe for payments.

Twilio for SMS.

Cloudflare for caching/DNS.