I agree with endorsements for Trello (for small teams), JIRA/Grasshopper for large ones (where you have a dedicated project manager who will manage JIRA, don't go there unless you are at that scale, or have a company full of process-loving engineers). Asana is another that I see a lot out there, but I feel it does a worse job at managing a backlog in the traditional Agile sense, and I find the interface a lot less intuitive than Trello. (Trello is slowly ruining theirs by trying to push it mainstream with weird stickers and crap, but you can ignore it....for now at least....)
The hardest part about templates for backlog management is the Epic/Story issue. That is, how to do mark epics, and divide them into stories at the right time, while still carrying forward the Epic identity into the story. Pivotal finally launched a whole Epics area a few years back that tries to address this head-on instead of forcing it through tags, etc.
Don't underestimate the value of a set of index cards as a model. If you have a co-located team, it can be quite charming, effective, and provides a high density of information as your team starts to use the cards in various ways. Search around for teams using physical index cards, you'll find lots of tips.