The main one is Zapier which can connect just about anything, but we also
use Mailshake which is an awesome yet very inexpensive emailing tool with
fantastic API connectivity. We also run our own server to read/write
directly through the Hubspot API. There are several other services we use
such as gmail, google docs, mysql, ring central, and several other
third-party apps. Pretty much anything that has API access can be connected.