Late last week, Minecraft developer Jens Bergensten let loose a pre-release version of the game's "Adventure Update" for dedicated players to try out early. It's also provided a means for the team at Mojang to gather some helpful feedback on bugs from the public, allowing them to ensure that the official release of 1.8 -- expected this week -- will be as stable and bug-free as possible.
If you want to play the version update now, you'll need to download the new .jar file and put it in your .minecraft/bin directory in your %appdata% folder. If that doesn't mean anything to you, this thread on the official forums gives some basic instructions on how to do so. Bear in mind that the pre-release version of 1.8 is a little unstable right now, with some reporting poor frame rates and other crashes occurring when doing simple things such as using a furnace.
That doesn't mean you shouldn't check it out, however, as the new world generation algorithms offer some spectacular landscapes and underground complexes for you to explore. And, of course, the creative Minecraft community has already been hard at work putting together some entertaining constructions -- check out the Duck Hunt video by Reddit user Sethbling above, for example.
Speaking on Twitter, Bergensten says that the current status of 1.8 is that he is "trying to fix the reported bugs of pre-1.8." He doesn't yet have an estimate of when this process will be complete, but Mojang's last guess was that it would be released some time this week -- possibly even today depending on the quantity and severity of bugs reported from the pre-release version.