Version 2.2.8 is the latest stable version; it was released in December, 1998. This is also the latest release version.
Briefly explained, -stable is aimed at the ISP or other corporate user who wants stability and a low change count over the wizzy new features of the latest release. At the moment, these versions are one and the same, but it shouldn't be long before the -current branch is polished enough for general release.
This is not to say that a 3.0-STABLE release is unusable for business services, and many people who need some 3.0 specific feature (newer compiler technology, faster networking code, etc) have decided to take a chance with it with very good results. We simply do not wish to "certify" 3.0 as mission-worthy until it's been released as 3.1-RELEASE in February, 1999.