Coda Hale is a Rails developer from Berkeley, CA, who's developed a ton of useful plugins. I'll leave the descriptions to Coda:

content_cache

Cache rendered views within the context of your layout, allowing you to mix dynamic content with cached static content.

dollars_and_cents

Stores prices as integers in your database, but access them as floats.

dynamic_session_exp

Allows you to configure Rails to serve up session cookies with expiration dates relative to the current time, instead of some fixed date in the future.

http_caching

Allows your Rails application to take advantage of the caching mechanisms built into HTTP 1.1 (i.e., 304 Not Modified return code). Inspired by a Rails cookbook entry (manuals.rubyonrails.com/read/chapter/62).

rails_environments

Simple methods for determining which environment Rails is running.

rails_rcov

Find your tests’ blind spots easily with Mauricio Fernandez’s rcov code coverage tool and this set of dynamic Rake tasks.

responsible_markup

A series of assertions for use in Rails functional tests which allow you to check for valid markup, unobtrusive Javascript, and other hallmarks of web professionalism.

simple_http_auth

A quick, clean way of adding HTTP authorization to your Rails application. No need for complicated backends, gigantic user models, or other unneeded complexity. Just you, a password prompt, and whether or not someone should be allowed in.

usps_countries

Replaces the Rails countries list with a list of countries to which the United States Postal Service can send mail. (If you’re concerned not with accurately representing people’s geographic self-identification, but rather where in the hell they want a package sent, this is for you.

xhtml_content_type

xhtml_content_type allows you to set the default MIME type for rendered .rhtml views to application/xhtml+xml if the client supports it, and only falling back to text/html for older clients.