SendGrid (http://sendgrid.com) - transactional emails (sending digital issues to subscribers).
MailChimp (http://mailchimp.com) - newsletter.
Linode (http://linode.com) - VPS hosting.
Harvest (http://getharvest.com) - invoicing (for corporate customers + advertisers).
FetchApp (http://fetchapp.com) - digital delivery (for single issue purchase). Previously used E-Junkie.
PayPal - payment gateway (sadly, one of the only choice for Malaysian).
Gumroad (http://gumroad.com) - I use this as a 'PayPal alternative' for customers who wish to pay directly with their credit card (and refuse to have anything to do with PayPal).
Pivotal Tracker (http://pivotaltracker.com) - project management for HM's backend app
ODesk (http://odesk.com) - finding and managing my remote team (currently in the size of 4).