I do not use Windows for work anymore, and have no hardware/space/time to maintain this. The file has been removed, but the article is here for archival purposes.
While I cannot vouch for the integrity of the builds, this page seems to provide x64 PuTTY builds.
TL:DR; Do not use this unless absolutely necessary. While to this day the official website does not provide 64-bit binaries, what is here is old, and quite likely is missing some critical security fixes.
This is a old post that has been migrated. While the content is probably very obsolete, in case someone needs it, it's here. Also makes great filler for a empty blog.
Original text copied from the official PuTTY website
PuTTY is a free implementation of Telnet and SSH for Windows and Unix platforms, along with an
xterm terminal emulator. It is written and maintained primarily by Simon Tatham.
The latest version is beta 0.60.
LEGAL WARNING: Use of PuTTY, PSCP, PSFTP and Plink is illegal in countries where encryption is outlawed. I believe it is legal to use PuTTY, PSCP, PSFTP and Plink in England and Wales and in many other countries, but I am not a lawyer and so if in doubt you should seek legal advice before downloading it. You may find this site useful (it's a survey of cryptography laws in many countries) but I can't vouch for its correctness.
This is UNOFFICIAL Windows x64 build of PuTTY, made for my personal purposes due to the lack of a official 64-bit build provided from the official distribution channel. This has not been throughly tested by a large amount of people and may (will) have problems.
If you encounter any 64-bit specific problems, please send me a note with reproduction steps and I will take a look. (As for now, there aren't any crash reporting facilities to handle this in PuTTY.)