On the IMAP front, Opera now supports special folders like sent items, spam, and trash. It also better handles duplicate items in Gmail -- such as those which appear in all mail and under your custom labels.
Opera 11.10 now partially supports the HTML5 File API as well, which means your favorite Web apps (like Gmail) may soon begin adding Opera to the list of supported browsers.
You can download the latest Opera 11.10 snapshot for Windows, Mac, or Linux from the official release post.
Update: The Opera 11.10 RC is now available, as pointed out by SlashZaku in the comments. Thanks!