On Monday afternoon, Apple posted Mac OS X Leopard 10.5.5 Update for PowerPC and Intel Macs. It is recommended for all users and is 136 MB in size.
The update is described in Apple support document HT2405. The update can be conducted from System Preferences -> Software Update or by using a standalone installer, referenced in the support article above. The standalone installer is considered a better approach.
The Update includes the contents of Security Update 2008-006 which is described in a deparate document, HT3137.
Apple typically posts the standalone installers to the Apple Downloads Page shortly after it appears in Software Update.