I bought a year's subscription to the online version of the Oxford English Dictionary just before Christmas, (it was on special), and it's providing me with hours of fun.
The thing I think I like best about it is looking up etymology. For instance, I wondered about the word 'dog', which, when you think about it, bears no similarity to any word referring to the kind of animals that 'dog' describes in any other European language. Where did 'dog' come from? Well, it turns out nobody knows.
The same is true of 'girl' and 'boy' and 'bird', all of which are also unlike words denoting similar things in other European languages.
More interestingly still, (stifle those yawns up the back there), 'dog' belongs to a whole group of words of unknown provenance - in this context, the editors of the Oxford English Dictionary invite readers to compare 'frog', 'hog', 'stag', and 'pig', as well as the Old English words 'sugga' (see Haysugge - hedge sparrow) and 'wicga' (earwig?) and even, possibly, 'teg' (a sheep in its second year [should a sheep manage to survive beyond its second year, is there a word for that, I wonder - but I digress]).
This may not be your idea of fun, of course, but I haven't time to worry about that - I'm in too much of a hurry to get back to the dictionary site where I'm hoping I'll discover more about 'fun' itself - the word that is and where exactly it hails from