If you moved entire folders around, but left the folders themselves intact (i.e. same structure they had before), you can simply right-click (In MacOS X, it might be something like Command-Click, I am not sure) the folder in the Library module's left-hand panel, and select "Find missing folder..." Simply browse to the folder's new location, and you're done.

If you did a radical reorganization, where the contents of folders or even original folders themselves are NOT intact, then you don't really have any option. If you intend to make that kind of reorganization, it is actually best to use Lightroom to do it, rather than the operating system. Lightroom is a little slower, but it will automatically keep track of everything. Additionally, Lightroom is capable of moving information with multiple threads, so you can have half a dozen move operations going on simultaneously and it does not seem to slow it down (I've had at least 10 simultaneous moves occurring at once in the past, and LR never skipped a beat...each move operated at the same speed, and they were only marginally slower than moving via the OS.)