This is a sample of a simple requirements matrix using Google Sites. You can implement the same thing in Sharepoint, which is much more capable. Or you can use an excel spreadsheet. It is pretty basic, but you can see how it would be useful. Requirement management tools are available, but only use these if you have the time to set them up. They can be very laborious and restrictive, and if the job is small to medium, more trouble than they are worth.
It is important to design your requirements management system before you start, even if you are using a configurable tool. Spend some time in classical db design before you start loading it up with data.