Google appears to have finally noticed that Mac users would like to sync their desktop calendars with their online ones: they’ve released (in beta form) Google Calendar CalDAV support. By following their instructions, you can set up synchronization between your OS X Leopard calendar and your Google one.
Though it works, the initial release is rough. Among the current issues are that it doesn’t work with other CalDAV-enabled software, it doesn’t support To Dos, and it doesn’t sync email reminders or alarms. You might want to look at the free Calgoo or the $65 Spanning Sync instead. In any case, I recommend backing up your data before experimenting.