rmtoo is a free and open source requirements management tool.
rmtoo uses a different approach than most other requirements management tools: it comes as a command line tool which is optimized for handling requirements. The power of rmtoo lies in the fact that the development environment can handle the input and output files - there is no need for a special tool set environment.
Example: if you need to handle baselines (and there often is), rmtoo can be configured using a revision control system (e.g. git). The revision control system can handle different revisions, baselining, tagging, branching and many other things extremely well - there is no reason to reinvent the wheel and making it less efficient.