If you use Github then you could do worse than use the Issue tracking functionality which is built into the site. It's simple but works for even relatively big project, we use it on the Unplugged Controls project for example.
When you commit your code which is related to an issue, if you prefix the issue number with a hash (pound sign) then the commit will automatically be linked to the issue for future reference.
If you prefix the # with the word fix then the issue will automatically be closed. So a commit might be:
Changed function getUserName() to fix #123
This will automatically close issue number 123 when you sync with Github.
Just a quick tip, but it makes life so much easier!