Running 10.5.8 it is up to date.
Note, there is a difference between update and upgrade:
Software update, upgrade--what's the difference? - http://support.apple.com/kb/HT1444
OS X: Updating OS X and Mac App Store apps - http://support.apple.com/kb/HT1338
Reference releases; update vs. upgrade
Mac OS X v10.1, 10.2, 10.3, 10.4, 10.5, 10.6 and OS X Lion themselves are not free updates, they are reference releases, also called upgrades.