The Devs have been working on the next version, 2.5 . They have said the release will be late due to all the major work in integrating Eclipse Smart Home and redesigning their build system.
They just built the second unstable build, 2.5M2, for distribution but still have a minor glitch or 2 before it is widely available.
2.4 is the latest stable build.
The stable version has normally been updated every 6 months. 2.4 was released in December. The 2.5 release has been delayed due to all the unexpected work this release cycle.
They released 2.5M1 (Milestone 1) testing in April and planned on monthly milestone builds. They just built 2.5M2 today but are having issues getting it to the normal Linux repos. After all issues have been resolved in the testing builds, 2.5 will get released.
In between there are unstable nightly snapshots but everything is not expected to function in those builds.