by the way, accidentally, apropos, by chance, by the bye, incidently, inconsequentially, noncoincidentally, inconsequently, coincidentally, more...

deliberately, intentionally, purposely, consciously, willfully

by the way, by the by, by the bye, more...

speaking of which,   as a side note,   on a side note,   as a byproduct,   by circumstance,   might i add,   as a side effect,   said in passing,   while you're at it,   oh by the way