I've intentionally excluded low-hanging fruit like "could of" from this list. That's too easy.
Myriad of
Ditch the 'of'. Myriad possibilities. Myriad errors. Myriad irritations.
Biweekly
Fortnightly.
Purposely
Until recently, I didn't even believe 'purposely' was a word. "Deliberately", "intentionally", "purposefully" or "on purpose" had always sufficed.
Could care less
Wish I could.
Irregardless
(Posted without comment)
Overwhelm
Overwhelm is a verb that people use as a noun. I hate it. Apparently, it has been used as a noun since the 16th century, so shows you what I know, but man I hate it.
Melancholy
You're feeling melancholic. There's something melancholic about the song. I'm filled with melancholy that dictionaries don't support my view that we should never use melancholy as an adjective or adverb.