- ravel = unravel
- flammable = inflammable: The source of "flammable" is well-documented. "Nonflammable" exists only because people started using "flammable."
- press = depress: As in buttons or keys on a keyboard. Occasionally I get authors trying to sneak "depress" in when "press" will work just fine. Some people just can't stand one-syllable words.
Can you think of any more?