If you're a developer who has been using the Golden Master build 13A598 of OS X 10.9 Mavericks, you may want to do a clean install because Apple has just released a new version. As 9to5Mac notes, the new build is 13A603 and as of yet no one has found any differences between the two besides the build numbers, though there are likely some small bug fixes.
It is rare that Apple releases a new GM build of an OS after seeding the first GM build, but it has happened in the past. Of note with this build is it will not show up in Software Update in the Mac App Store if you have the prior GM build installed. You'll need to do a clean install of the 13A603 Mavericks GM build to get it on your system.
Apple is expected to announced the download date of OS X 10.9 Mavericks for consumer use at this Tuesday's event.