UPDATE: Nothing to see here, folks. The only big Android announcement during the keynote is that Google will start selling a Galaxy S4 equipped with stock Android (4.2) via its Play store.
At Google I/O 2012, Google introduced the world to Android 4.1, or Jelly Bean. It later updated Jelly Bean to version 4.2. Now at Google I/O 2013, it looks like Google will be updating Jelly Bean again to version 4.3.
The Verge reports that a page on Google's Android developer Web site referenced Android 4.3 before being removed. It was still available via cache for a while with a listing for security enhancements in the newest version of Android, but nobody was actually able to catch what those security enhancements were.
Regardless, we still know next to nothing about Android 4.3. Knowing it exists is a start, but there's not been much in the way of leaks in regards to new features. Some are suggesting that the latest update will just be a minimal update with support for Bluetooth Low Energy and better OpenGL for Embedded Systems 3.0. The latter jibes with other rumors suggesting that Google will announce some new gaming initiatives for Android at the conference.
It's important to note that Android 4.3 will most likely still be called Jelly Bean. Android boss Pundar Sichai already said that Google wouldn't be showing off a new version of Android so a small update to Jelly Bean is the only thing that makes sense. Those looking for Key Lime Pie (or Kandy Kane) will have to keep waiting.