When I owned a 360:

2 play and charge kits (my 2nd controller came bundled with the console)
1 wifi adapter

Yes, I have bought a few:
Battery charger - very convenient
Wireless headset - much better than the wired set as I found it cancels out echo. Best add-on I 've bought.
Wireless adapter - only item I don't use as I found the wired ethernet/homeplug solution much better.
VGA lead - slight improvement in picture quality but bought mainly to free up my component input.

Edit - Also have the camera but rarely use this. 

Yeah, but only the following:
Play & Charge kit (a green one, as I bought a green controller too)
HD DVD player (50 Euro well spent, it gives regular DVD's much better picture than the PS3 blu-ray player does IMO)
And that's about it!

Extra controller
HD-DVD player cheap
Extra 20gig HD cheap

If the HD-DVD drive and 20gig HD weren't being liquidated cheap I wouldn't have bothered.

I forgot to mention I have a charge kit as well with two chargeable battery packs.

And my XBOX, which by some definitions could be viewed as a multiplayer peripheral :P

2 wireless controllers ($65.00 for them on Amazon), A used 20g hard drive for 20 bucks, rechargeable AA which I use for everything, so I can't really count those. And, my most expensive accessory, 42" 1080P LCD TV.

One WiFi adapter (that works surprisingly well for where it is), one charge and play kit, and one battery pack for my second controller.

Oh, and I also later bought a 120gig hard drive when NXE and game installs came about.

THe only thing i bought worth mentioning is the wifi adapter. I feel stupid for having bought it though. Ethernet cable is more reliable and far, far cheaper

You should only feel stupid if you have your router two feet from your 360, otherwise, it's a good thing.

My 360 is stuffed inside an entertainment center with gadgets above and below in a cabinet (and it does overheat!).  The Wifi adapter is pushed all the way back in the cabinet and it still works great so yeah, it's pretty expensive, but it's also pretty goood.