Security fix: the Windows PuTTY binaries should no longer be vulnerable to hijacking by specially named DLLs in the same directory (on versions of Windows where they previously were). See vuln-indirect-dll-hijack.
Windows PuTTY no longer sets a restrictive process ACL by default, because this turned out to inconvenience too many legitimate applications such as NVDA and TortoiseGit. You can still manually request a restricted ACL using the command-line option -restrict-acl.
The Windows PuTTY tools now come in a 64-bit version.
The Windows PuTTY tools now have Windows's ASLR and DEP security features turned on.
Support for elliptic-curve cryptography (the NIST curves and 25519), for host keys, user authentication keys, and key exchange.
Support for importing and exporting OpenSSH's new private key format.
Host key preference policy change: PuTTY prefers host key formats for which it already knows the key.
Run-time option (from the system menu / Ctrl-right-click menu) to retrieve other host keys from the same server (which cross-certifies them using the session key established using an already-known key) and add them to the known host-keys database.
The Unix GUI PuTTY tools can now be built against GTK 3.
There is now a Unix version of Pageant.