Another great example which has not been talked a lot (because it lacks the cool factor) is the inclusion of Howl in Apache Geronimo and ActiveMQ so that these products can support transaction persistence and recovery.
Another milestone I'm waiting for is the integration of JOTM in Geronimo. Jean-Pierre Laisne and I were at the origin of the change of license of JOTM from LGPL to BSD to make it possible to use JOTM in Geronimo (ASF does not accept integration of LGPL code in their codebase). I believed that the change of license was a good opportunity for both JOTM and the J2EE Open Source community in general because ObjectWeb and Apache communities would then be able to contribute to it instead of wasting resource and time to duplicate a transaction manager.
To be honest, JOTM has not attracted a lot of contributors yet but I expect it to change once it's integrated in Geronimo and can reach a wider audience among J2EE developpers.
Brian Behlendorf used the term coopetition (cooperation + competition) to describe the Apache and ObjectWeb relationship and that's a quite accurate description (no needs of lawyers between the two, only software layers). At the end, what counts is that both communities gain from collaborating and sharing code.