It didn't drop last Friday as rumoured, but the Mac OS X 10.5.7 update is now available, so fire up Software Update and get your 449MB update on.
This release includes mostly bug and security fixes, including some strange ones like, "Addresses a situation that may cause issues when logging into Gmail." Who knew that was an issue? Check the official update page for more specifics.
About the Mac OS X 10.5.7 Update [Apple]