Aegisub's x64 builds for Windows are EXPERIMENTAL.

The following modules are not compiled:
- automation 3

There are still some problems with avisynth - It will work with a x64 build of avisynth (google for it) on the program folder and of vsfilter on the csri folder, but audio loading doesn't seem to work (will load from PCM Wave, though), and subtitles preview on the style editor won't work. Dummy video also won't work, due to lack of csri interface in vsfilter.dll.

FFMPEGSOURCE WON'T WORK.

You will need this to install these runtimes for Aegisub x64:
http://www.microsoft.com/downloads/details.aspx?familyid=bd2a6171-e2d6-4230-b809-9a8d7548c1b6&displaylang=en

And these for ASSDraw3 x64 (sorry, I used different compilers):
http://www.microsoft.com/downloads/details.aspx?FamilyID=EB4EBE2D-33C0-4A47-9DD4-B9A6D7BD44DA&displaylang=en

To install, just drop the exe alongside with the latest x86 build and replace avisynth.dll, directshowsource.dll and vsfilter.dll for x64 builds.