The most important Agile tool

Having just completed a review of our latest release, a lot of the team complained about the lack of some of the tools and processes that we've become used to in recent times. These shortfalls were mainly due to the fact that we're integrating with a new team but it led me to think about which single tool I'd hate to be without.

The answer is Continuous Integration, we use CruiseControl by the way.

We get a lot of spin-offs for doing this, especially as we're a distributed team, the main one being an implicit communication tool, everyone is using it, so we can all see who's doing what. Other benefits are that we also using several build metrics, so I believe the quality level is kept high, which gives a much warmer feeling, especially when the pressure is on.

Which Agile tool or process would you hate to be without?