I don't use anything... but if I did, it would probably be vinegar... bonus would be it cleaning your washing machine at the same time! Other fabric softeners are actually really bad for your washing machine.

Question tho with vinegar - when using it with MCN's the warning was there that vinegar would deteriorate elastic and things... wouldn't it do the same to clothes if used regularly? I don't think I'd ever use it every wash as a regular softener.