Those of you on the networks of better universities probably have access to that finest of English dictionaries: the Oxford English Dictionary. A bit of the colourful history of the thing can be learned from the entertaining book The Professor and the Madman. Perhaps the most notable thing about the OED is that it doesn’t simply seek to define all words in the English language, it also seeks to identify when they were first used in a particular sense. As such, it constitutes a wonderful history of the language itself.
Here’s a special bonus for people using Firefox or Safari. Follow these instructions and you can add a new button that, if pressed, opens a popup window that lets you do an OED search. Also, if you select a word in a brower window and click the button, it will automatically look it up.
Like online access to the OED in general, this will only work if you are physically connected to the network of an institution that subscribes, or you are using a virtual private network (VPN) to access such a network.