At Mind Your Language, we have no problem with different spellings being used for US English. We just don't want to see people deploying the American spellings in an Australian context, or mixing the two together in a single document. And so we come to artefact.
Antiquites picture from Shutterstock
The Macquarie Dictionary, our constant oracle on these matters, lists 'artefact' as the main spelling, but also allows 'artifact'. In the US, 'artifact' is the more common spelling.
Around these parts, we always stick with Australian spelling, so 'artefact' is what we're inclined to use. If you feel inclined to deploy 'artifact' -- perhaps because you've spent too much time reading US sites about overclocking -- then make sure you use it consistently. Don't switch from one to the other. That makes you look ignorant.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.