The AR15 platform is the big boys 'Erector Set', remember those? A bunch of pieces of metal you could assemble into anything your imagination could dream up. With the basic starting blocks of a box stock rifle you can go in so many directions it can make your head spin. With a starting point of a 5.56 rifle you can add to that a dedicated .22 lr upper or just a conversion kit, from there the caliber choice is up to you. 6.8, .308, .50, you name it and within reason you can do it. Stocks, foreends, optics and so many accesories you could drive yourself to the poor farm by trying to keep up with the weapons race.
I currently have 5 AR's each configured differently, with a couple of uppers to play with amongst them. One of my favorites is a 9mm dedicated rifle, little recoil, cheap ammo (relatively speaking), my wife and niece both just love this thing. I've even seen a black powder upper advertised, why one would want that is beyond me, but they are there. Want a cross bow? Well, get an upper with that feature. Even wierder than the BP but whatever floats your boat I guess.
Not taking anything away from our site but AR15.com will keep you up late at night just looking and dreaming of some ideas.
Make the plunge and hang onto your wallet!
RD