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