>>> http://en.wikipedia.org/wiki/Comparison_of_issue_tracking_systems
BUT please remember that a tool is only as good as the strategy and thoughts behind it...do not become a slave to a stupid constraining software BUT OWN THE SHOW!!!
make your RE strategy and thinking modify the use of the tool so it is really SUPPORTING the intended workflow and gives all involved the energy to work according to the workflow/process (so that the decided tool-driven workflow is at least as good as if not using the tool at all)
Do not become a slave to a tool!
Many people do the "tool-dance" (many hours of hard work to get the tool to work in the environment) and after these hours they are so "comitted" that their Common Sense has left the building
Do not use a hammer to write and take notes!
As an IREB Super Hero, I am confident that you will use your chosen tool as a Super Hero Ray Gun!
To protect the good project and to support the good cause of developing an excellent product!
