kdenlive in Ubuntu 12.04, I was not able to export videos in H.264 and other formats out-of-the-box. Instead, I would get "output format not supported" errors when trying to render the videos, even though libx264 was installed.I came across this multiple times already and was scratching my head every time, since enough time had passed since the last time I dealt with this. I could not easily remember what Ubuntu package was missing. Now it goes in my little knowledge base for next time, hehe.
The package needed is
libavcodec-extra-53:$ sudo apt-get install libavcodec-extra-53
