CoTweet ( http://cotweet.com ) - Shared Twitter account management. Great for support and guerrilla marketing.

Uservoice ( http://uservoice.com ) - User feedback and support management.

Dropbox ( http://dropbox.com ) - File sharing - we share the business dropbox with our individual dropboxes to share files.

Droplr ( http://droplr.com ) - Screenshot capture and sharing. Also allows for file sharing.

As mentioned by others:

GitHub ( http://github.com ) - VCS

Google Apps ( http://www.google.com/a ) - Email and documents.

Pivotal Tracker ( http://www.pivotaltracker.com ) - Project management and issue tracking.

Wordpress ( http://wordpress.com ) - Blog.

These are very specific to Rails, but extremely useful:

New Relic RPM ( http://newrelic.com ) - Deeply integrated Ruby on Rails app monitoring and performance measurement. Immensely helpful for troubleshooting and analysis, including slow query detection/explanation, etc.

Hoptoad ( http://www.hoptoadapp.com ) - Rails exception monitoring and alerting.