Right now you can get the update to 10.5.5 which Apple says fixes, well, a lot. See below for all the listed fixes.
General
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 (see this article for more information).
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.
Address Book
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.
Disk Utility and Directory Utility
Improves reliability when rebuilding a software mirror RAID volume in Disk Utility.
Improves reliability of server status displayed in Directory Utility.
iCal
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.
MobileMe
Improves overall sync reliability.
Improves Back to My Mac reliability.
Time Machine
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).
