Trello
I am currently using Trello for very simple projects. We also use it for any Stack Exchange planning, from snacks you need to buy this week to Trello cards with user stories.
Trello basically gives you cards and a virtual card. These are the basics of agility.
Rally
In the past, I was a very happy Rally user who is completely free for 10 users - a good team size.
The rally gives you stories, burnouts, tasks, sprints, quality and defect tracking.
Other tools
source share