Rails Rescue Handbook – Mike Gunderloy shares bits of his years of experience in a very well written ebook. How to approach a new project when taking over, what to pay attention to, all the things that can possibly be done wrong, tools to use. Highly recommended reading for anyone in the Rails business.
rack-bug – Debugging toolbar for Rack applications implemented as middleware. Can conveniently show elapsed time, break it down, database query info, cache hits, templates being rendered. Useful for inspection and profiling.