check out ATVFlash and/or patchstick.ca
both will allow you to buy and download software that will 'jailbreak' the ATV by plugging in a usb flash drive. Very easy to get to that point.
I believe that both allow you to install Boxee, which is supposed to give you access to Hulu. However, I was never able to actually get Hulu to play a video. I could get to the interface, which looked like garbage, but nothing would actually play. YMMV.
Personally, I don't think it's worth the effort for Hulu - just get a Roku and be done with it.
The one thing that I thought was worthwhile on the ATVFlash version is nitoTV, which allows you to connect to file shares (aka NAS), and play most file formats directly - the only thing that I can think of that won't play this way is DRMd files.
Really, both of these patch kits open up some cool features on the original ATV, I just found that after going ohhh, ahh, for about 5 minutes, I went back to watching TV. I'd rather browse on my iPhone than on my TV...