While OS X 10.8.5 Mountain Lion build 12F36 was released on Wednesday, the other two seeds mentioned in Apple's email are new.
OS X Server Preview 7 for Mavericks is designated as build 13S357e and comes with focus areas similar to the recently seeded OS X 10.9 Mavericks Developer Preview 7. Apple asks developers to concentrate their efforts on graphics, Wi-Fi, sleep from wake and iBooks PDF imports.
The latest Windows Migration Assistant reaches version 18.104.22.168, and is designed to help users migrate data from a Windows PC to OS X. The seed notes do not mention any major issues with the software.
Developers can download all three updates through Apple's Developer Center or Software Update.