My latest refcard, which was published this week covers HTML5 - one of the hottest topics going. Of course, the specification isn't complete yet, but a lot of the features can be used in the latest versions of web browsers. If you're unfamiliar with what's involved, take a look through the card and you'll see that it is the biggest evolution of browsers since we moved from text based to including images.
The standout feature of HTML5 is the Canvas element. The API looks pretty compehensive, covering a whole lot of graphic operations. Two parts of the specification that I think are really useful are the Geolocation API and the storage options.
I'm looking forward to seeing where the HTML5 specification brings us. Some believe that HTML5 replaces the need for OS-specific mobile applications. I still think that there is a place for native mobile applications to take full advantage of a device's features.
Although it's still early days for HTML5,plenty websites have already taken advantage of some of the features. Perhaps you could share your favorite HTML5 enabled sites in the comments section?