I've recently been interested in Google's Ngram Viewer regarding differences in spelling of some words such as travelling vs traveling, busses vs buses or learned vs learnt. Of course, there are many reasons for such spelling differences, however, it might attract the attention of students to see how the common use of some words (they don't really have to be problem words in spelling, could be any word) has changed over time. This might help them to understand the idea that language is like a biological organism and that it changes over time and that there is not just one exact fact or true answer to every question they may have. Also it might be helpful for us teachers as well to explain them why they see one spelling more than the other.
Here is an example looking at the phrase 'Merry Christmas' versus 'Happy Christmas'. Upon closer inspection to the dates, it appears that the peaks and troughs are associated with major wars: World War One, World War Two, the Korean War, and the Vietnam War.
Aside from the historical context of use, Google Ngrams can also let you explore cultural contexts. For example, Google Ngrams lets you specify the 'type' of English, so if we look at British English, you can see that 'Happy Christmas' is relatively more common.
As opposed to American English, where 'Happy Christmas' is quite rare.
While on the subject of 'merry', NGRAMs is also an interesting tool to look at synonyms, e.g., merry, festive and gay. A good example of how a word like 'gay' can take on different meanings over time.
Here is another example of prepositions that are used with the word 'weekend'. While the focus of this line of inquiry was on prepositions, the surprising fact that was revealed was that the word 'weekend' didn't exist in the English language until 1940. This would suggest that the working week used to be six days until then. So the history of a word gives lots of insight into human history as well.
But, what is even more useful, is the ability to make your search 'case sensitive'. So, for example, you can compare the same word in different cases, e.g. 'But' as opposed to 'but'. One at the start of a sentence, the other as a conjunction within a sentence. It is nice to be able to give an answer to students who insist that it is possible to start a sentence with 'but'. Yes, indeed. But be careful.
All in all, I think it's a nice tool to play around...One can even start discussion son target words and say "Why do you think this word was popular in the 1890s but not very popular now?"