On Mac, at least in Netscape or in Word,
Option + "g" = ©

Option + "n" and then the letter (e.g., "n") = ñ

Option + "u" and then the letter (e.g., "o") = ö

Option + "e" and then the letter (e.g., "e") = é

Option + "i" and then the letter (e.g., "o") = ô

Option + "a" = å

Option + "o" = ø

Option + "w" = *

Option + "p" = *

Option + "©" and then the letter (e.g., "n") = ñ

Option + "c" = ç

Option + "n" and then the letter (e.g., "n") = ñ

I actually forgot where I found these -- probably in the MS Word manual. But it's pretty easy to discover them by playing aroung with the Option and Command (Apple) keys and various letters (or by using "KEYCAPS" on an older machine). There is also an INSERT SYMBOL option in the INSERT menu.

I have found that these usually show up as entered on the internet and in emails.
(Having said that, I will probably find that it won't work in this post!)
Genie