Lead Source Campaigns: Directly link your Leads/Contacts to a Campaign, and then have campaign specific emails send through your automation tool like MailChimp.
Open Cases on Accounts and Contacts: Your customer service team could send out emails after X amount of days on an open case to make sure the issue was resolved. Alternatively, you could pause your marketing messaging if there are open cases on an account since they are probably not looking to buy while they're resolving issues.
Opportunity Product on Account: If you have multiple products, you can cross-sell, up-sell, or personalize recommendations based on what they've bought before.