Recently I have been playing Left4Dead, HL2 (w/ Synergy mod), and a couple other FPS games with a sort of unusual control setup. I have been using my mouse as usual, but instead of using my keyboard I hook up a 360 controller and use it for movement, jumping, crouching, and other functions that i can't fit on my mouse buttons. It's very easy to find a comfortable position for playing when you only have to worry about positioning your mouse pad.

What I'm wondering is if there are any wii nunchuck-style peripherals (basically, only the left half of a regular controller) for the PC that are designed for this control method... If not, someone should make one!

It's not the same. I have use both the nostromo and nunchuck. They both feel completely different. I'm thinking of a device that looks and feel like the nunchuck that could be plug into a share PS/2 connection in the back of the PC. You could easily get a splitter that could split the keyboard PS/2 connection, they already have that for barcode scanners.

Anyway, all this "PC nunchuck" has to do is simulate the key stroke, e.g moving the nunchuck stick up would send a key stroke of "w" to the PC. In terms of the game software, it would think that is coming from the keyboard. Obviously the "PC nunchuck" is programmable so you could change what ever key you want. Most FPS games have "w" as forward, "s" as backward, "a" as left, and "d" as moving right. You could program the "PC nunchuck" to simulate those key stroke with the movement of the stick. In terms of the C and Z buttons of the Wii nunchuck, we still could keep that but now it's programmable therefore the C button could simulate a "spacebar" key stroke, which usually means jump in FPS. The Z button could simulate a "c" key stroke, which is usually crouch.

I would add 2 more buttons to the side of the "PC nunchuck" that could be program either to simulate a single key stroke each or both buttons could be use to cycle through sets of programmable sequences. One button would cycle the sequence forward and the other backward. For example we could program the sequence to simulate the numbers "1", "2", etc. up to 10 key stokes. In FPS games number keys usually change weapons, therefore you could cycle the weapon you want, by pressing the forward or backward buttons.

You'll still use the mouse as normal, but the keyboard will be replace by the "PC nunchuck".

I'm in the process of trying to make a prototype. I'm using a Microchip PIC to simulate the key strokes, but it's harder then I thought, although I'm inching closer.