Agile maintenance releases
Damon Poole has an interesting post about agile development "by mistake". He argues that the most agile release type in not-aware-of-agile environments is the maintenance release. It typically improves quality a lot and adds a number of select features that are really needed by the users of the system. This is consistent with my own experience. Often a major release is followed by a number of maintenance releases that is developed in a very agile way. It seems that the proximity of a release is making a team more agile.