The first is Mac mini EFI Firmware Update 1.2. This update "improves compatibility with the latest Apple memory kits on Mac mini computers." This update is only needed for Mac minis introduced in March 2009 or later.
Next, there's an update to Mac OS X Leopard Server, Mac OS X Server 10.5.8 Update v.1.1, which includes general operating system fixes, as well as specific fixes for a few other issues. This update is available as a Combo or Standalone update.
Finally, there's a firmware update for SuperDrives. This update "will eliminate the noise made by the optical disk drive during system startup and wake from sleep on your Mac." This update is only needed for iMacs and Mac minis introduced in March 2009 or later.
As always, these updates are available through Software Update on your Mac, or you can download them from Apple's Support Downloads page.