It's just too hard to spell check your text by hand (or it requires an unreasonable amount of time). Before you actually get a spell checker, you might not even realize how many typos you produce. I have just downloaded today the FireFox RC1 that includes a web form spell checker. My first target for typo hunting was an intranet wiki I am working on. I feel that I won't ever go back to a browser that does not provide such a feature natively.
My second target for typo hunting was the source code of a Asp.Net application I am currently developing. Since Visual Studio 2005 does not provide a native spell checker (shame! such feature should have been implemented years ago), I have used the MailFrame CodeSpell product. CodeSpell has a really nice VS 2005 integration. On the minus side, the dictionary is a bit light at the present time (
aspx not being part of the default dictionary for example) and I did experience of a few crashes while playing with the custom user dictionary.
A funny aspect of applying this source code spell-checker was to discover that 3rd party [*] XML documentations present in my .Net solution were no less typo-crippled than my own code. If automated spell-checking is not part of your development process, well, it should.