The 'Tiger' release, 5.0, boasts a raft of enhancements to both platform and language. Improvements for programmers include Collections, metadata, and C language features such as enums and variable arguments. It's hard to imagine that Java has reached its tenth year and only just got the equivalent of the C language's va_arg, but there you go.
The platform side has new systems management and profiling APIs and OpenGL support, and a new look and feel, called Ocean. The "native" look and feel has been spruced up to make it look more er, native. Sun claims VM start-up time has been improved too.
Off you go, then.