One of the tools Ann mentioned was MSN AdCenter's Keyword Mutation Tool. Despite its grotesque moniker, it's a great for optimizing internal site search. You don't have to be an MSN advertiser to use it, either.
Basically, what this tool does is generate common keyword misspellings for you. So instead of trying to forecast them yourself, or waiting until someone types these errors in your search box and gets a "results not found," you can adjust your internal search to include these possibilities.
It takes some playing around with though, for example I got no results for "appliance" but a few for "appliances":
You'll also want to manually test these misspellings to make sure they work (for your most popular or important items / keywords). If your ecommerce software search function uses fuzzy logic, your search should be able to recognize variations of keywords. But it's always worth a spot-check for your critical terms.