If you use emacs on a Linux or non-Mac Unix machine, it probably already has iSpell installed and configured so you could spell check in emacs (eg, use M-$).
In Mac OS X, you'll do well to use ASpell instead via CocoaASpell.
In Windows...I don't know. There is an ASpell ported to Win32, but it is horribly out of date.
If you know a good emacs spell check solution for Windows, let me know in the comments below. Thanks.