I'll start by admitting that I'm a bit of an Apple fanboy. I have a Mac Mini, wife has a Macbook and I use a Macbook for work.
I have a Lacie NAS that I bought at the lacie.com outlet for pretty cheap. Works fine for PC and OSX but it's pretty loud. I also use 802.11n at home. Last week I finally installed my Apple Time Capsule (1TB) and it's great. All 3 Macs now backup automatically to the Time Capsule and all computers access this hard drive. I know others who use it with their PCs and Macs at home. Good solution but not $100.
I don't think you'll find great $100 NAS solutions. The best one I've seen is drobo. But it's very $$$ and only attaches via USB. You can get the Ethernet adapter but it adds cost. You still have to attach your own HDD. However, it's probably the most powerful for the home because it's doesn't really need a PC to config. Another option would be HP's new MediaSmart servers. They've had really good reviews, highly rated on Amazon as well.
