Mac OS X 10.5.5

Mac OS X 10.5.5 was a feature and bugfix update for OS X Leopard released September 15, 2008. The Apple KBA states:

General Address Book Disk Utility and Directory Utility iCal Mail MobileMe Time Machine
 * Includes recent Apple security updates.
 * Addresses stability issues with video playback, processor core idling, and remote disc sharing for MacBook Air.
 * Addresses an issue in which some Macs could unexpectedly power on at the same time each day.
 * Resolves a stability issue in TextEdit that could be found when accessing the color palette.
 * Improves Spotlight indexing performance.
 * Fixes an issue in which contacts might not sync properly with PalmOS-based devices.
 * Improves iPhone sync reliability with iCal and Address Book.
 * Includes improvements to Active Directory.
 * Improves Speech Dictionary.
 * Fixes Kerberos authentication issues for Mac OS X 10.5 clients that connect to certain Samba servers, such as Mac OS X Server version 10.4.
 * Includes extensive graphics enhancements.
 * Addresses stability issues that may occur when creating a Smart Group.
 * Resolves a printing issue with address cards containing information that spans more than one page.
 * Improves reliability when rebuilding a software mirror RAID volume in Disk Utility.
 * Improves reliability of server status displayed in Directory Utility.
 * Updates iCal to more accurately handle repeating events.
 * Improves performance when choosing meeting attendees.
 * Resolves an issue in which the "Refresh All" option may be dimmed ("grayed out") in the contextual menu for certain calendars.
 * Fixes issues with read-only calendars.
 * Addresses an issue that prevents an invitee from moving an event to a different calendar.
 * Resolves an issue with syncing published calendars.
 * Addresses performance issues related to displaying IMAP messages.
 * Resolves an issue with SMTP settings for AIM, Compuserve, Hanmail, Yahoo!, and Time Warner Road Runner email accounts.
 * Addresses stability issues that may occur when dragging a file to the Mail icon in the Dock.
 * Addresses an issue with the "Organized by Thread" view in which the date does not appear when the thread is collapsed.
 * Resolves an issue in which RSS feeds could temporarily disappear from the sidebar.
 * Improves Mail robustness when sending messages.
 * Improves reliability when saving drafts that have attachments.
 * Improves overall sync reliability.
 * Improves Back to My Mac reliability.
 * Improves Time Machine reliability with Time Capsule.
 * Addresses performance issues that may affect initial and in-progress backups.
 * Fixes an issue in which an incorrect alert message could appear stating that a backup volume does not have enough free space.
 * Time Machine can now back up iPhone backups that are on your Mac, as well as other items in (~/Library/Application Support).

External References
Apple: About the Mac OS X 10.5.5 Update