One of the notable additions to this release is now there's support for EGL/GLES2 rendering, which makes it possible to utilize this open-source Adobe Flash/SWF player in some embedded environments, e.g. on ARM devices. The GLES2 back-end allows Flash to work with reasonable speed on ARM hardware and in other cases where a desktop OpenGL stack is unavailable.
For those not interested in the EGL/GLES2 rendering, Lightspark 0.5.1 restores support with playing YouTube videos. Flowplayer is also now compatible with Lightspark. Additionally, there's crash fixes and other enhancements to this new version. Vimeo has also seen bettered support by this release.
More details can be found in the Lightspark 0.5.1 release announcement. Meanwhile, in the official Adobe Flash Player world, Flash 11 should soon be released with mainline 64-bit support, improved video support, and other exciting enhancements.