On our way back from our anniversary brunch at the Hotel MacDonald (delicious!), Peach asked me about the difference between the words 'complimentary' and 'complementary'. I didn't really know the exact answer for her, but I told her I'd look it up. And since it's an interesting and useful tidbit of information, I thought I'd post it here. (From ThincTanc.)
Complementary with an 'e' means 'to complete'; complimentary with an 'i' means 'to praise' or 'freely offered'. Therefore, it's 'complementary colours' and 'compliments to the chef'. Interestingly enough, if someone offers you a 'complimentary software package' that means it's free (or as free as those things get), but if someone's offering a 'complementary software package', it means that it's designed to work with pre-existing software - but it's likely going to cost you.