In honor of the fact that Yahoo! keeps using duel when they mean dual, I thought I’d list off a few more words I see that can or are confused with one another:
-ball vs. bawl -won vs. one -cannon vs. canon -versus vs. verses -sheath vs. sheaf (as I mentioned, I made this mistake in my own novel. Fortunately, caught it on the fourth pass through…)And since this is a really short post, I’ll provide a few more helpful links:
Marion Zimmer Bradley’s Pet Grammar Peeves (She may be gone, but she still can teach us a thing or two. Pretty basic list, but worth knowing.)
Purdue OWL: Verb Tenses
PCW on Formal and informal (A good discussion about how that English you learned in school isn’t necessarily the English you should use in your writing. But it could be. Bottom line: Use the full range of options in your writing based on what is most appropriate for the situation.)
M.H. Lee 
People shuddered, not shuttered. A mistake I and a Pulitzer Prize winner both made in our books! (According to my husband, though, the other author made it twice.)
I also misused alter (when I meant altar) on a website I used to have.
And I don’t know if it was done on purpose, but in “The Hunger Games,” everyone goes to the Capitol, but that word means a building. The geographic seat of government is a capital.
Those are some good ones! At some point I’m going to consolidate the three or four posts I’ve made on this so far and will definitely add these to the list.
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Two words that I continually confused and will never again are dessert and desert. A fifth grade student of mine told me that dessert has two s’s because you always want more!
That’s another good one. Every time I use one or the other I think I’m using the wrong one. But from now on I’m going to remember that trick about dessert having two s’s because you want more.