In the International preference pane, add Keyboard Viewer, Character Palette, and, if you want hardcore stuff like Chinese/Japanese characters, Unicode Hex Input. One of those three options will give you the ability to enter any character you need.
Basic characters that require alt codes in Windows, like em and en dashes, accented vowels, n with a tilde, bullets, trademark, copyright, registration, and so on, can be typed with simple two- and three-key combinations on a Mac.
ñ option-N then N
é option-E then e
å option-A
option-2
© option-G
shift-option-hyphen
and so on