For some reason, I like words about words. These are not so useful in casual conversation, but awareness of them lends insight into the evolution of language and its structure. Following here are some of my favorites:
retronym - A word that has been changed to allow greater specificity for that word. e.g. "television" had to become "broadcast television" with the advent of "satellite television" and "cable television". See here for more examples.
eponym - a word naming a place, discovery, era or other item after its discoverer. Most interesting to me are the corporate eponyms that become the general term to describe something, e.g. Kleenex for tissue, Band-Aid for adhesive bandage, Xerox for copy machine. A more complete list is available here
capitonym - a word with the same spelling but different meaning based on capitalization. e.g. August vs august (the month vs the adjective)
heteronym - two words spelled identically but with different meanings and pronunciation. e.g. tear (rip something) vs tear (water from the eye)
meronym - Using part of something to refer to its whole. e.g. Threads (slang) as a meronym of clothes.
hyponym - a word of more specific meaning than another. A subset. e.g. Blue is a hyponym of color.
hypernym - a word of more broad meaning than another. A superset. e.g. Color is a hypernym of yellow.
antonym - a word opposite in meaning to another. e.g. love is an antonym of hate.
homonym - words with the same pronunciation but different meanings. e.g. There, their and they're are all homonym's of each other.
metonym - a word/phrase used as a substitute for something else it is closely associated with. e.g. "Madison Avenue" is a metonym for advertising.