I've been thinking of posting something on this for a while. Because I can't be the only one that is enough of a geek to care about this.
Everyone already knows about http://www.dictionary.com, of course.
But one of the radio stations here also has a mini-program they've started on the origins of phrases and the archives are available online.
http://www.ckua.com - I'd post the site for just the program, but I can't manage to get the url. (I hate frames sometimes.....) Just go to the bottom where it has a pull-down menu named "Select a Program" and choose the program called "Expressions"
(It's also a pretty good radio station in general and I'm happy that I finally have a computer that Windows Media Player will work on..... Yay! I will include a bit of a disclaimer that they're doing a fundraiser right now, so they are a little more talkative than usual.
And oh yes.... this is the station that I first heard "Nobody Here But Us Chickens" on first...)