Mac OS X 10.5.8 available through Software Update
The latest update to Mac OS X (which is sent to every Mac owner) automatically upgrades your Safari 3 install to Safari 4.
This is excellent and exactly what should happen on all operating systems. There is no reason not to upgrade your browser, but every reason that you should. Some people cannot be bothered to go find the download though, so bringing it automatically to people like this is perfect. Well done Apple!