The new keyboards (introduced in 2007), wired and wireless, are both very thin and made from aluminum with white plastic keys. As has already been stated, the wireless version does not have a numeric keypad. You can find these on Apple's webiste:
http://www.apple.com/keyboard/
The previous keyboards (intoduced in 2005? or 2003...I forget), which also came in wired and wireless, looked more like a "traditional" keyboard, with "normal"-size white keys. In fact, the whole keyboard was white, except for an outer and bottom casing of clear plastic. Both wired and wireless versions look nearly identical except for, of course, the lack of cord on the wireless.
Before that white keyboard there was another that looks decently similar, although with some extra space behind the function keys (I believe there was an extra USB port there) and, I believe, one fewer function key, and the function keys were grouped into four rather than going straight across without gaps as they now do. There was also a black version of this keyboard that was later discontinued, leaving only the white version.
...and before that, I think there was another black keyboard, and before that, all the iMac G3 keyboards, and before that, you'll be taken to the pre-USB land of ADB keyboards.
But yeah ... the only keyboard you can find from Apple anymore is the newer aluminum ones. (For a while, they sold the white generation before it as refurbished, but I haven't seen them in the refurb store for a while now, and I assume they are all gone.) If you search eBay or Google, however, you can definitely find some of the white generation before (or before that...or however far back you want to go

), probably both new and used.
I'm thinking about getting the previous-generation white Bluetooth keyboard myself, since I would like an external keyboard to use along with my external monitor for my MacBook...that I am waiting until September to buy and hope I'm not let down (I just sold my old one

)...and I am not a fan of the lack of numeric keypad on the new one.
Hope this helps!