I love the fact that web tools offer spell check. My favorite web site development tool, Dreamweaver, has spell check for when you are creating web pages. Firefox has it too (I see that it does not like the word Dreamweaver). Many of these tools are even getting better with correcting common grammar errors too, but they can't catch everything!
One of my readers recently submitted her blog site that deals with common errors, especially homophones which many tools will not correct.
I make an effort to go back and review my work after a day or two. I hate when I find stupid errors and hope I correct them before others see them. When blogging or developing web sites, always recheck your content after a few days to make sure you have it right. Sometimes it's little things that will turn your audience away from your site.