Indivisible Guide - The inspiration for Indivisible Chicago. 

Call My Congress - A tool to find out who your Members of Congress are and how to reach them.

A Guide to Calling - A guide to getting through the anxiety and making those first calls. (from Christopher Grace)

The 65 - Legislative phone numbers, weekly calls to action, and calling scripts for specific issues (an evolution of "We're His Problem Now," below). 

Countable - An app that helps you learn about bills up for vote in Congress and much more. A great tool to understand who is voting for what. 

Rise When We Fall - Sign up to receive action alerts (i.e. - calling Members of Congress) through text message.

GovTrack.us - Tracking the actions of Congress and bill that pass.  You can follow them on Twitter as well.