I use Stack Overflow quite often to get quick answers to questions I have while developing.
I use Cloud9 as my preferred Rails development environment. It saves me a lot time and trouble compared to developing locally (I run Windows).
Cloud9 IDE uses Ubuntu, so indirectly I built apps with Ubuntu.
Skype is one of the better methods I've found for IM communication, and most of my clients have it. Mostly use it when there is a need for screensharing.
Mailgun is my goto when I need to send email from within a web app.
Google drive is rather useful for shared editing of documents and specs.
All my apps have been deployed using Heroku. I've found it very easy to use.
GitHub is very nice for team coding.
I use Trello for organizing projects. Typically I break a project up into sections, add and prioritize tasks for each section. For largest projects, I'll break it into multiple trello boards.
Heroku recommends PostgreSQL, so I use it and it seems to work well enough.
New Relic is awesome for monitoring app performance and status.
Rails is built on Ruby, so in the end Ruby is Rails.
Slack is very nice for team chats.
Rails and Git go hand in hand. My .profile is filled with Bash aliases for Git.
I use a bit of jQuery here and there for AJAX.
As a web app developer, all of my web apps have been built with Rails.
Bootstrap is great for getting an MVP working when function matters more than form and MVPs are what I do.