JIRA is a great tool but it's hard to set up and for small teams it can be overkill. We built Zube to provide an Agile workflow right out of the box so you can get up an running right away.
Zube is also deeply integrated with GitHub and everything is synced both ways in real time. So even if some of your developers want to stay in GitHub, everything is still up to date.
To me, this two-way sync'ing is the killer feature of Zube (from my short usage so far), along with its Trello-like card metaphor for issues as a value-added layer.
Just having the real-time filtering on issues is a huge win. Change a filter, see your updated list. Excellent.
Zube is also deeply integrated with GitHub and everything is synced both ways in real time. So even if some of your developers want to stay in GitHub, everything is still up to date.