What exactly do you mean by 'decompiler'?
By that i assume you mean take a video file and convert it into another format?
There are a number of tools available; but they vary depending on the type of input file....
i'd firstly point you in the direction of MPEG Streamclip from Squared 5
(Link here) this is a good all round video conversion tool and can output to a number of pro level formats (i've also found it beneficial to install a custom codec pack that i found via digg.com - it helps you to open some odd Windows files like XviD and DivX codecs;
link here) The best thing about MPEG Streamclip is its free!!
Another tool i use for quick conversion is VisualHub
(link here)... this is quite good for format to format conversion and is very quick indeed...
I hope this is of help to you!
Bomski