Many email newsletter software vendors offer transactional email support, which gives companies the ability to include promotional messages within the body of transactional emails. There are also software vendors that offer specialized transactional email marketing services, which include providing targeted and personalized transactional email messages and running specific marketing campaigns (such as customer referral programs).
Twitter has Twitter Lead Gen Cards, which let you generate leads directly within a tweet without having to leave the site. A user's name, email address, and Twitter username are automatically pulled into the card, and all they have to do is click "Submit" to become a lead. (Hint for HubSpot users: You can connect Twitter Lead Gen Cards to your HubSpot Forms. Learn how to do that here).