Indexes or indices?

I was reading an article on Friday about the volatility in the stock market, and it referred to "the major indexes" and I got to wondering whether that was grammatically correct. There are a handful of English words that have irregular plurals. For example, did you know the plural form of appendix is appendices when you are talking about the section in the back of the book, but it's appendixes when you are talking about multiple internal organs? Matrix takes the plural form matrices but I don't think you could properly use that form when referring to multiple copies of the Keanu Reeves movie. The Wall Street Journal apparently considers both forms acceptable. Wiktionary claims that indexes is more common in the US and indices is the British form. But does common usage make a word correct? Certainly that's not true initially, though it does tend to become a recognized correct usage if it persists. Irregardless is an example of a word that has come to be accepted, though Merriam Webster's online dictionary still claims it's not a word.