Gnash

If you are using Ubuntu 12.04 on an older computer with a Semperon CPU, you will soon find that Flash is not working and therefore Youtube videos will not work either.

Adobe Flash built with SSE2 flag

This is because Adobe has build the recent Flash versions with the SSE2 flags enabled, yet many Semperons lack this particular flag.

$ cat /proc/cpuinfo
vendor_id : AuthenticAMD
cpu family : 6
model : 8
model name : AMD Sempron(tm) 2600+
...