I use a site/tool called Aiveo. Couple of my buddies made it. It's free for teams up to 5 users and it's super simple. I love it because of its simplicity/clean look, kinda reminds me of how simple to use Slack is for IMs.
I've tried many of them such as Trello, MS Team System (now called Visual Studio Online I believe?), YouTrack, JIRA, FogBugz, BugZilla.. Honestly so many of them other than Trello are far too complicated and often obtuse. Trello I liked a fair bit, but I still like Aiveo far more. Bonus points, they're very quick to implement changes and updates on Aiveo for features you request.