I put together a catalog of the key tools we are using:

Development Environment

RVM Managing multiple applications, multiple versions of Ruby and gemsets.
SmartGit GUI client for git.
Sequel Pro GUI client for MySQL.
Transmit GUI client for Amazon S3.
Lighthouseapp Issue tracking.
github Source code repository.
git-flow Source code branch management. Jeff Kreeftmeijer wrote a nice overview:
Why aren’t you using git-flow?


spec, capybara, selenium-webdriver, factory_girl Unit and integration testing.
tddium Continuous testing. You can find more details at Continuous Integration in the Cloud.
Sauce Labs Cross-browser testing.
litmus Email testing.
httperf Load/stress testing.

Production Environment

RightScale / Amazon EC2 Servers An overview of the environment is available at Managing Uptime on Amazon EC2.
Amazon CloudFront CDN
Custom built RightScale RightScripts Software release process. You can find out more at Deploying Ruby applications in the cloud.
delayed_job, cron Job management. More details are available at Running distributed cron jobs in the cloud.
RightScale Server monitoring.
New Relic Application and availability monitoring.
Airbrake Exception management.
PagerDuty Alert management.
SendGrid Email delivery.

What tools are you using?