Codementor is an online marketplace connecting developers with experts for on-demand 1:1 help via screen sharing, video and text chat.
We use a hybrid of everything. It's mainly Google Analytics and Mixpanel, and we also write our own back-end analytics for some stuff as well.
We also use Zoom for video calls. It has a remote control, which Skype does not.
We use Slack, not Skype anymore, for video.
Stripe is extremely easy to integrate. However, they don't provide ACH payouts--they require that the other side be a Stripe user in the US--though they've been slowly rolling that out internationally.
We use UserVoice, and we also looked into Zendesk. We chose UserVoice because Zendesk seemed more bloated.
We used to use MailChimp but ended up switching to SendGrid a few months ago (gotta show some love for the Techstars family).
It's very lightweight and easy to use.