OpenNMS, like many open-source projects, has 2 parallel development tracks:
- Stable: rock-solid and production-ready, bugfixes-only
- Unstable: bugfixes and new features
When the unstable release is ready, the current stable release will be deprecated and the new codebase will take its place. A new unstable is created, and development continues.
All the available documentation can be found here.
A high-level feature list is always available at New and Noteworthy.