PuTTY 0.61 supports SSH-2 authentication using GSSAPI, on both Windows and Unix. Users in a Kerberos realm should now be able to use their existing Kerberos single sign-on in their PuTTY SSH connections.
On Windows: PuTTY's X11 forwarding can now authenticate with the local X server, if you point it at an X authority file where it can find the authentication details. So you can now use Windows PuTTY with X forwarding and not have to open your X server up to all connections from localhost.
On Windows: the Appearance panel now includes a checkbox to allow the selection of non-fixed-width fonts, which PuTTY will coerce into a fixed-width grid in its terminal emulation. In particular, this allows you to use GNU Unifont and Fixedsys Excelsior.
On Unix: the GTK port now compiles with GTK version 2, which is generally shinier and in particular provides access to client-side scalable fonts.
Some Linux distributions have been shipping pre-release versions of GTK 2 PuTTY for years, so this won't be a surprise to anyone using Unix PuTTY or pterm via Debian or Ubuntu. But this is the first official release containing that functionality.
A small but important feature: you can now manually tell PuTTY the name of the host you expect to end up talking to, in cases where that differs from where it's physically connecting to (e.g. when port forwarding). If you do this, the host key will be looked up and cached under the former name.
Assorted optimization and speedup work. SSH key exchange should be faster by about a factor of three compared to 0.60; SSH-2 connections are started up in a way that reduces the number of network round trip delays; SSH-2 window management has also been revised to reduce round trip delays during any large-volume data transfer (including port forwardings as well as SFTP/SCP).
Support for OpenSSH's security-tweaked form of SSH compression (so PuTTY can now use compression again when talking to modern OpenSSH servers).
Support for Windows 7's new user interface features. The new Aero window management should now play nicely with PuTTY's complicated window resize handling, and Windows 7 jump lists are now supported so you can launch saved sessions directly from the taskbar.