CheckedTwice is an online gift registry that makes it easy for users to exchange gifts the way they want to, with the people they care about.
We also use Segment (in addition to Mixpanel and Google Analytics), which I can't recommend enough.
Mixpanel is well-featured, but expensive. It's tough to use that over the free Google Analytics.
I love Mandrill. It's such a simple API, and it's cheap.
MailChimp is more hands on (than Mandrill), but it works well.
It's free and it has good integrations.
I used QuickBooks for accounting. The integration with the online payroll service made things very easy.
... a shout out for Meldium. It’s great for password management.