If you want to type Spanish on your Mac and are used to Windows, you may run into problems figuring out which keys to use for accented characters and the inverted punctuation marks. That's because, out of the 6 commonly-used Spanish keyboard layouts provided on the two platforms, only two (Mac Spanish ISO and Windows Spanish) are the same. How this situation came about, I have no idea. You can see graphics comparing the six here.
My understanding is that many Spanish users prefer the Windows US International or Latin American layouts. For the US International layout, a version can be downloaded from this page, and for Latin American (winlatinamerican.keylayout) you can go to my iDisk.
Of course you can also type Spanish on the normal US layout: The deadkey for the acute accent is Option + e and for the tilde it is Option + n, with ¿ at Option + Shift + / and ¡ at Option + 1.