Jim Weirich and Joe O'Brien came up with something cool today at the TDD w/Rails studio. If you're using RCov to keep your test coverage total, it's useful to realize that rcov is just a command-line tool.
You might be using Selenium or Watir for browser testing. If you want to use RCov to figure out how much of your app actually gets run by your Selenium or Watir tests, it just requires using a few command-line options:
rcov script/server -o server_coverage --rails -x config/boot.rb -- -e test -p 3002
Red: rcov command
Blue: script/server args (the -- means "end rcov args")
Black: OS X command to view the output
This will start a Mongrel instance, wrapped by RCov, which will find out how much of the code actually runs, and give you a detailed, color-coded report. Obviously, this technique can be very useful if you're dealing with a legacy Rails code base. Any time the path of execution through the code is unclear, using tools which can easily determine that path makes your life easier. For anybody who wishes Ruby had kick-ass debugger support, congratulations: now it does. In two lines of Unix. (And unlike even the Smalltalk debugger, this debugger only works if you use automated testing.)