The ultimate goal of any website is to have users perform some sort of action. That might be to make a purchase, or click a banner, or bookmark your site. Whatever it is you want the user to do, it is your content and your users' perception of your professionalism that will entice them to perform that action.

There's no reason to make things difficult for the user, and bad spelling and grammar both give a bad impression and make a site less of a pleasure to use. Non-visual user-agents, like screen readers, will trip over bad spelling. Bad grammar will put a user off, slowing down their use of your site.

There really is no excuse for it either. It doesn't take long to check text over. Most word processing packages will even do it for you if you want them to.

Check your spelling.

It doesn't take long, and the benefits are enormous. Bad spelling gives the distinct impression of unprofessionalism. Would you trust your money or personal details to a company that couldn't be bothered to spell check their own site? Of course not. You would find yourself another site - one which gave off a more professional air. Most users are the same - most are wary of the internet and giving out information, and will only do so if completely comfortable.

Every half-decent word processor includes a spell checking function. There are plenty of websites that offer spell checking. There really is no excuse for bad spelling on a site.

Check your grammar.

Grammar is a little more tricky than spelling. I should know - mine is awful. However, grammar is important on a website, as much so as with other written mediums. The better your grammar, the easier a text will be to read. It will flow better, and the user will find it easier to scan and understand. Checking of grammar can be done easily enough through most word processing programs (though inevitably it will not be a perfect check), and by reading text.

Normally, if checking grammar, I'll print out the text I am checking first. Reading text on a screen is ok, but most people tend to read too quickly and skip things. You might find you pick up more errors if you read from paper.

Hire some help.

If you are serious about making sure your copy is perfect, hire a professional to check it for you. There are plenty of copywriters out there, and their experience is usually invaluable. Best of all, it means that you are not spending your own time on spelling and grammer, something few people enjoy, which leaves you more time to get on with more serious things.