FreeBSD 10.0 went into alpha earlier this month and it was followed by a second alpha release one week later. FreeBSD 10.0 Alpha 3 ended up being skipped due to build issues, so now we're already up to a fourth FreeBSD 10 Alpha in less than one month's time.
FreeBSD 10 has tons of new features including but not limited to the long-awaited AMD Radeon DRM/KMS driver, LLVM Clang replacing GCC, and a new virtualization hypervisor.
FreeBSD 10.0 Alpha 4 has various changes over Alpha 2 including updates to various packages, new drivers, epoll support in Linuxulator, LLVM's LLDB is connected to the build but disabled by default, Xen PVHVM para-virtualization support in the GENERIC i386/amd64 kernel configuration, a new libcxxrt release, and it removes the armv6eb architecture.
It still will likely be some time before the FreeBSD 10.0 Alpha release is ready to ship, so until then check out the FreeBSD 10.0 Alpha 4 announcement if you're interested in helping out by testing the BSD operating system.