If you are okay with a minimum requirement of OS X 10.6+, and can suffice with “read-only” access to the stream of events, you can install a global event monitor in Cocoa:
Cocoa Event-Handling Guide: Monitoring Events.
If you need to support OS X 10.5 and earlier, and read-only access is okay, and don’t mind working with the Carbon Event Manager, you can basically do the Carbon-equivalent using
GetEventMonitorTarget(). (You will be hard-pressed to find any (official) documentation on that method though). That API was first available in OS X 10.3, I believe.
If you need read-write access to the event stream, then you will need to look at a slightly lower-level API that is part of ApplicationServices > CoreGraphics:
CGEventTapCreate() and friends. This was first available in 10.4.
Note that all 3 methods will require that the user have “Enable access for assistive devices” enabled in the System Preferences > Universal Access preference pane (at least for key events).