Log4r is a comprehensive and flexible logging library written in Ruby for use in Ruby programs. It features a hierarchical logging system of any number of levels, custom level names, logger inheritance, multiple output destinations per log event, execution tracing, custom formatting, thread safteyness, XML and YAML configuration, and more. Log4r works really well, so please take advantage of it right away! :) All versions since 0.9.2 have been stable and backward-compatible. The code is stable enough that updates are infrequent and usually only for adding features or keeping the code up to date with Ruby.