DNSai - Free SPF, DKIM and DMARC checkers, plus DNS/WHOIS lookups and email-header analysis; no signup.
Welcome email generator - Type a SaaS's address and get the emails it should send, free and with no sign-up.
Campaign Cleaner - Spam checker, Font Color Adjuster, Blacklist Link Checker, Broken Link Checker, CSS Inliner, HTML Sanitizer, Non-ASCII replacement, Font-Size Adjuster, and more.
Add-to-calendar links in email - Free guide to adding "add to calendar" buttons that work in Gmail, Outlook and Apple Mail: why JavaScript widgets fail in email, how to build Google/Outlook/Yahoo links and a hosted .ics by hand, and links vs .ics attachments.
Cold Email Checker - Free, no-signup check of a cold email's length, reading level, spam words, links, questions and ask; runs in the browser, open source (MIT).