capybara-webkit depends on a WebKit implementation from Qt, a cross-platform development toolkit. You'll need to download the Qt Framework to build and install the gem.
If you're on OS X, the installer at the above link will set up everything you need. You can download just the framework, as the full SDK isn't required to build the gem.
If you're on Ubuntu, you can install the libqt4-dev package.
On Linux platforms, capybara-webkit requires an X server to run, although it doesn't create any visible windows. Xvfb works fine for this.
Add the capybara-webkit gem to your Gemfile:
The capybara WebKit driver was written by Joe Ferris, Tristan Dunn, and Jason Morrison from thoughtbot, inc.
The names and logos for thoughtbot are trademarks of thoughtbot, inc.
capybara-webkit is Copyright © 2011 thoughtbot, inc. It is free software, and may be redistributed under the terms specified in the LICENSE file.