Apple has been beta testing OS X 10.8.4 for some time now and it has now released a new version of OS X 10.8.4 with a build number of 12E55 to developers. This particular update for Mountain Lion focuses on Safari, Graphics, WiFi, and Windows File Sharing. There is even added support for Mac computers with 802.11ac WiFi chips.
Apple has been beta testing OS X 10.8.4 for some time now (this is actually the eighth beta) and as such it is likely nearing public release. Considering that Apple plans to unveil OS X 10.9 at the WWDC 2013 in June the fruit company should release it in the next couple weeks.
If you are a developer you can head on over to the Mac developer portal and start downloading it.