valuable vs. invaluable - These words can have slightly different meanings depending on the situation, although they can be used interchangeably without much difference. Valuable things have a known value, and invaluable things are those "of value too great to be estimated". A good way of remembering this: invaluable items have value that is inestimable.
flammable vs. inflammable - These words have identical meanings. Inflammable actually comes from the root word inflame, and it is not supposed to be a negative prefix. The word inflammable is being phased out, and rightly so; it just isn't worth the safety risks involved.
regardless vs. irregardless - Again, no change in meaning. Considered nonstandard as it is a double negative; the -less suffix already puts it in the negative, no need to add a negative prefix.
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