Lightspark 0.4.2 RC2 is compatible with H.264 Flash videos on YouTube, faster video presentation via optimizations to the SSE2 based video packer, and there is audio support via PulseAudio. Alessandro Pignotti, the lead developer of Lightspark, says there are still a few more features planned prior to the official release of Lightspark 0.4.2.
In Alessandro's announcement he also mentions that a Lightspark 0.4.3 feature is the ability to fall-back to using Gnash to handle some Flash/SWF files not currently supported by Lightspark. Gnash is the Free Software Foundation's Flash Player. Beyond the source being available, packages of Lightspark 0.4.2 RC2 are available for Ubuntu/Debian and Fedora for easy testing.