Calvary refers to the hillside on which Jesus was crucified.
Cavalry is a group of highly mobile army troops.
Eminent means “prominent, famous"
Imminent means "about to happen"
The pair that bugs me the most (right now) is titled vs. entitled.
If something is “titled” it means that it received such a title, either by the author or by someone else. Entitled, on the other hand, means that a person has rights to something.
It is never all right to spell "alright" as one word.
Jabberwock - a monster appearing in a poem by Lewis Carroll.
Jabberwocky - the name of that poem.
I'm in the middle of reading a modern novel based on Carroll's "Alice in Wonderland", in which Alice is being hunted by a monster that has yet to make an appearance. So far, it's an interesting read, but every time that the monster is referred to as "The Jabberwocky", I wince.
This isn't the first time that I've encountered the same confusion between name of poem and name of monster.
Adhering strictly to the category of English usage error that Mr. Willis specified: the misapplication of similar sounding words, I believe the most common one sees is the confusion over affect and effect.
If this thread migrates into common misapplications of English grammar of any stripe, there's not enough time in life for me to list even just the ones I hear and read regularly.