Spelling mistakes are not an inevitable part of life. It may seem easy to miss errors but they stand out like a sore thumb to your readers.
There are some simple tools to help with spell checking. The problem with relying on automated tools to spell check for you is that you can end up with correctly spelt but wrong words. The only way to fix that type of mistake is to read your content carefully. That said, automated tools will help you to pick up on the majority of problems and therefore they shouldn't be ignored.
I like to use the Firefox Spell Checker add on by Gaurang033. You need to set the language and dictionary preferences before you first use it, otherwise it wont work at all. Once it is configured and working, it highlights spelling mistakes across the whole page, not just in text boxes.
Another one I saw today is an online service which you can try for free called CheckDog. This is really useful to use against an existing site, as it runs through each page on the site and checks them all.
It doesn't take too long to read through what you have written and make corrections, so there is no excuse for poor spelling. Making sure your text is free from errors will make it easier to read and more professional than a page with many errors.