I just disabled my iBook's keyboard with milk, and it will be a couple days before I can let it dry for 72 hours, figure out whether the damage can be cleaned up, order a real replacement, etc. I needed a USB keyboard right away.
There's a Circuit City nearby and I noticed that they sell the Logitech USB Keyboard for the PlayStation 2, model number Y-UC29, part number 967199-0100 (although mine says 867199-100), aka "NetPlay Keyboard," for just $19.99. I went and bought one, wondering what I'd say if it didn't work in the Mac and I had to return it.
No such problem, it works fine with the Mac out of the box.
The bottom row is ctrl, fn, *Logitech*, alt, space, alt, ctrl, arrows,
where the iMac has fn, ctrl, option, *apple*, space, *apple*, enter, arrows.
The space key is half a key narrower on each side, and the Logitech key is regular key width, compared to the apple keys' one and a half. "Alt" maps to "option" and the "Logitech" key maps to "command," aka "open apple," aka "four-leaf clover". Perhaps because of the funny graphic on that key, using it for keyboard shortcuts seems to happen naturally. The ctrl/fn/cmd/option rearrangement hasn't bothered me so far (but thinking about it, Emacs's undo, ctrl-option-hyphen, will be a little clumsier). Force-kill (option-cmd-esc) is the same (Logitech-Alt-Esc).
There is no equivalent to the iBook's enter key, even with fn pressed. The return key is labeled "<-+ Enter", but that's not the same, e.g. in spreadsheets (not that I use Apple's "enter").
The "fn" key turns some of the letter keys into a number pad with the same mapping as on the iBook keyboard. The arrows are in the same inverted-T but in a slightly different location. Fn maps them in the same way to PgUp, Home, PgDn, and End. There is a working Del key (forward-delete) that my iBook keyboard doesn't have. Fn-Del is Ins, but pressing it causes my browser (Firefox) to go nuts as I'm typing this. What's labeled "delete" on the iMac is in the same place but called "<- backspace". Esc, backquote/tilde ("`" / "~") and Caps Lock are in the same places.
Holding the F12 makes the CD eject. F9, F10 and F11 do their Expose things. CAPS LOCK WORKS, num lock doesn't seem to.
The keyboard is black, narrower than even the iBook keyboard, and not counting the frame, less high. The alphabet keys are of standard size and spacing. The number row and function key row are about 2/3 height, like the function keys on the iMac. This makes typing numbers annoying. The feel is a little drier and more clacky than the iBook's keys, but not by much, and besides I find the iMac's keys a little smooshy/slippery. One problem for me has been that unless I put the Logitech right on top of the iBook keyboard, my hands automatically go from clicking with the trackpad to using the dead iBook keys and I feel stupid. So, I plop the Logitech keyboard right on top of the iBook keyboard. Because the frame overlaps a little on the top and bottom, none of the iBook keys get pressed. But, the USB cable comes straight out of the top of the Logitech keyboard and some of it would loop up in front of the screen had I not taped it out of the way with clear duct tape.
(What a pinacle our civilization has reached with clear duct tape! Amazon's page has my review. Kevin Kelly's review. But I digress.)
Unlike the built-in keyboard, this one uses a USB slot, and unlike some USB keyboards, it doesn't have an internal hub and more ports to plug your mouse or camera into.
Maybe the 8-foot USB cable could be used to type while lying in bed with the computer on the desk or windowsill; I haven't tried that.
All in all a pretty good emergency spare for twenty bucks. The nearest-price competitor in the Circuit City cost $69, an ugly standard PC keyboard with separate number pad, arrow pad, etc. (I have never understood why people want keyboards like B-52 flight decks or whole alligator skins.) Logitech's own cheapest USB keyboard advertized for the Mac seems to be $29.95 ( Logitech's keyboard page ) and is in a similar cushioned aligator batchelor-pad multimedia-center style. Logitech's page for the Playstation USB keyboard lists it for $29.95 as well.