These four gems have helped my team improve the quality of our code. We weren't writing bad code before but these tools made it possible for us to take our code to the next level.
Flog helps you analyze the complexity of your methods. It looks at the number of assignments, branches, and calls in a method and creates a complexity score for each method. The lower the score the less complex the method is, generally making it easier to read, test, and refactor.
Flay analyzes code for duplication. It looks for structural similarities so things like variable names and whitespace won't keep it from detecting duplication in your code.
Roodi parses your code and gives warnings on several different code design issues. Roodi includes a few different measures of code complexity such as cyclomatic complexity, line count, and parameter count as well as some useful lint style checks.
Rcov is a code coverage tool for ruby, it shows you what code was run during your tests. While that doesn't guarantee that you tested all of your boundary conditions it does go a long ways in giving you confidence in your test coverage. My teams threshold for code coverage is 100%, anything less is treated the same as a failing test. If you are planning on using rcov with a rails app I would suggest the rails_rcov plugin.
Bonus FREE rails plugin
When my team runs tests on our apps we use rails_quality a plugin I wrote that runs all four of these gems during the testing process. Any output that is outside of the thresholds we have set is treated the same as a failing test. I would recommend this practice to anyone. It will make your code better and it pays off way earlier in the development cycle than you expect.