This exists and is what Hound is built on. Rubocop runs lint checks against your Ruby code: https://github.com/bbatsov/rubocop/ . The Emacs integration is great; it highlights failures in place. Hound looks interesting, but having Emacs/Vim integration gives you a tighter feedback loop, so you don't have to wait until Pull Request time to discover these issues.
You can also use linting tools as a build validation step (ours are invoked via things like `make myapp.pep8test`). These can also then be invoked yourself before making a commit. As jwinter says, this really tightens the loop when fixing trivial errors.
Almost all of the tools are very configurable in terms of which warnings/errors you want to ignore (e.g., let our max line length be 120 instead of 80), and give pretty good feedback (line numbers, exact error).
Another great benefit is that you have a neutral arbiter of what's the style for your codebase, which helps avoid ruffled feathers.
Check out the pomo gem. It works in a very similar way plus it adds a 25-minute timer for pomodoros. If you add your .pomo file to git would do the same integration.
I also recommend checking out Audiogalaxy. It works surprisingly well on 3G and is a great way to access your entire music/podcast collection from your phone. I use it all the time.
